Radio receiver for use in a radio tracking system

ABSTRACT

A radio receiver unit for directionally tracking at least one radio transmitter in accordance with the invention includes a housing containing a radio receiver including a directional antenna for receiving radio transmissions from the at least one radio transmitter; a display, which is electrically coupled to the receiver and fixed in position with respect to the housing, for visually displaying a strength of the radio transmissions received by the directional antenna; and a field of view limiter for limiting light emanating from the display to a field of view of the display when a user holds the receiver unit in the user&#39;s hand away from the body of the user, the field of view being limited to planes extending upward from a plane of sight extending from the eyes of the user downward and intersecting a horizontal plane extending from the user&#39;s waist substantially at arms length of the user.

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 08/394,268, entitled "Radio Tracking System and Method ofOperation Thereof", filed on Feb. 24, 1995, now U.S. Pat. No. 5,640,146,and is also a Continuation-In-Part of U.S. patent application Ser. No.08/394,267, entitled "Radio Tracking System and Method of OperationThereof", filed on Feb. 24, 1995, now U.S. Pat. No. 5,650,769, whichapplications are incorporated hereby by reference in their entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to U.S. application Ser. No. 08/568,416, entitled"Radio Receiver For Use In a Radio Tracking System and A Method ofOperation Thereof", filed on even date herewith, which application isincorporated by reference in its entirety.

Reference is made to U.S. patent application Ser. No. 08/386,060, filedFeb. 7, 1995, entitled "System for Wireless Serial Transmission ofEncoded Information", U.S. patent application Ser. No. 08/385,312, filedFeb. 7, 1995, entitled "Receiving Circuitry for Receiving SeriallyTransmitted Encoded Information", and U.S. patent application Ser. No.08/385,143, filed Feb. 7, 1995, entitled "Transmitting Circuitry forSerial Transmission of Encoded Information" all filed on Feb. 7, 1995,which applications are Continuations-in-Part of U.S. application Ser.No. 08/112,256, filed Aug. 26, 1993, entitled "Information TransmissionSystem and Method of Operation", now U.S. Pat. No. 5,446,759; which is aContinuation-In-Part of U.S. application Ser. No. 07/850,275, filed Mar.12, 1992, entitled "Low Power Information Transmission System HavingHigh Information Transmission and Low Error Rates and Method ofOperation" (now abandoned); Ser. No. 07/850,276, filed Mar. 12, 1992,entitled "High Speed, Low Power and Low Error Information Receiver andMethod of Operation" (now abandoned); and Ser. No. 07/850,487, filedMar. 12, 1992, entitled "Low Power Information Transmission andReceiving System Having High Information and Low Error Rates and Methodof Operation" (now abandoned), which applications are incorporatedherein by reference in their entirety.

Appendix

Attached hereto is a source code listing of 57 pages. Pages 1-50 containsource code used for control of a digital signal processor embodying theprocessor 402 of FIG. 14 for controlling the operation of a preferredreceiver 12 to be used in association with the present invention. Pages51-57 contain source code used for control of a digital signal processorembodying the control processor 106 of FIG. 3 for controlling operationof a preferred transmitter to be used in association with the presentinvention. The processor 402 which executes the source code of pages1-50 is a 68HC705C series. The processor 106 which executes the sourcecode of pages 51-57 is also a 68HC705C series. A limited license isgrangted to any one who requires a copy of the programs disclosedtherein for purposes of understating or anlayzing the present invention,but no license is granted for any other purpose, including the loadingof a computer with code in any form or language.

TECHNICAL FIELD

The present invention relates to radio tracking systems for locating amobile radio transmitter and for determining if the mobile radiotransmitter has moved outside of a set range measured from a radioreceiver and to mobile radio transmitters which transmit an alarm of auser of the radio transmitter to a radio receiver, and methods ofoperation thereof.

BACKGROUND ART

Parents are becoming increasingly concerned that their children may beharmed when they are out of their sight. Almost daily the media reportsevents involving small children being harmed when the small childrenhave wandered from sight of their parents. Furthermore, in today'sincreasingly mobile society families with small children regularly visitmalls, amusement parks and other public places where crowds of peopleare found which provide an environment where small children can beharmed or become lost or wander from sight of their parents because oftheir natural inquisitiveness, tendency to explore their surroundings,or their desires to be free from control of their movements by theirparents.

Devices are commercially available to limit or monitor movements ofchildren. Devices exist for tethering children to their parents. Furtherradio systems are commercially available which generate an alarm whenchildren move outside a radius from a radio receiver which receivestransmissions from a transmitter worn by children. The tethering deviceshave a limited restraint radius and create animosity between a child andthe parents. The radio systems have a fixed radius of approximatelyfifteen feet which is too small to permit useful monitoring if a parentdoes not wish to totally keep a child in sight and cannot be used fortracking.

Numerous radio tracking systems have been proposed which utilize radiocommunications to locate a mobile radio transmitter and/or to determinewhen a mobile radio transmitter carried by a person has exceeded a setrange measured from a radio receiver. These systems have one or moreradio transmitters which broadcast a coded identification of each radiotransmitter which is received by a radio receiver and processed todetermine the distance and, in some of these systems, the directionbetween each transmitter and receiver. See U.S. Pat. Nos. 4,785,291,5,115,223, 5,119,072, 5,245,314, 5,289,163, 5,307,053 and 5,357,259,Patent Application WO 87/06748, U.K. Patent Application GB 2182183A andJapanese Patent Application No. 64-311842. A wide range ofimplementations of radio tracking systems are described in theabove-referenced patents and published applications.

The determination if a mobile radio transmitter has moved out of rangefrom a radio receiver receiving an identification code of the radiotransmitter is accomplished in many different ways in these patents andapplications. Two ways which are described for determining if a mobiletransmitter has moved out of range are by determining if the receivedidentification code signal has dropped below a predetermined signalstrength or the received identification code signal has not beenreceived for an elapsed time interval.

Radio communication systems which are designed to determine when amobile transmitter worn by a person has moved outside of a set rangeand/or to track a person encounter severe problems because of (1)limitations of transmitter power imposed by the Federal CommunicationsCommission which limit broadcast power below 100 milliwatts, and (2)various environmental factors which cause interference, fading, orsignal attenuation of the identification code signal which isperiodically sent from the mobile radio transmitter to the monitoringradio receiver. The transmitter identification code signal may beseverely attenuated by passage through the bodies or body parts ofpeople or other structures in the line of site between the radiotransmitter and the radio receiver. The presence of people andstructures in the line of sight causes substantial attenuation of thetransmitted identification code signal which may cause theidentification code of the radio transmitter to be periodically orpermanently attenuated below the discrimination level of the radioreceiver causing a false indication that the mobile radio transmitterhas moved out of a set range and an inability to further track themobile radio transmitter.

Furthermore, natural fading phenomena, such as Rayleigh fading, which isa function of the transmitting frequency and the relative velocitybetween the mobile radio transmitter and radio receiver are severelyaggravated by low speed movement, such as when a child or patient iswalking with a transmitter attached to their person to facilitate theirtracking. These fading phenomena affect the determination if a set rangehas been exceeded and a direction determination of the transmitterrelative to the receiver. Additionally, other man-made interferences,such as electrical noise and multipath interference caused by buildings,can periodically cause the identification code signal transmitted fromthe radio transmitter to be attenuated to a level below thediscrimination level of the radio receiver tracking the transmitterwhich also causes a false indication that the radio transmitter isoutside a set range and/or the inability to track the direction of theradio transmitter movement relative to the radio receiver with adirectional antenna.

Error correction code may be transmitted in a frame of bits encoding theidentification code of the radio transmitter. One or more framesencoding the identification code of the transmitter may each contain aset number of error correction code bits which are processed by theradio receiver to correct minor bit errors such as one or two bits whichoccur within the identification code frame bits. One well known errorcorrection code for accomplishing this function is the BCH code.

The serial processing of the bits of frames which contain errorcorrection code is typically implemented with a series of EXCLUSIVE ORgates. When a number of bit errors in a frame exceeds the errorcorrection capacity of error correction code, the data within the frameis erroneous. The prior art methods of wireless data transmission do notpermit the recovery of valid data bits from a frame containing a numberof bit errors which exceed the bit error correction capacity of theerror code therein which error correction capacity, for most types oferror correction codes, is two bits.

The cumulative effects of mis-synchronization of a radio receiver toreceive transmissions from radio transmitters, Rayleigh fading, andman-made noise noticeably reduces the reliability of current digitalradio receivers to receive error free data. A gap in a data transmissionin excess of 1 millisecond may cause a radio receiver to terminate thereceiving process. In a situation of tracking a radio transmitter with aradio receiver which receives a periodic digital transmission of theradio transmitter's identification code, termination of the receivingprocess results in the correct identification of the radio transmitternot being received. As a result, the transmission from a radiotransmitter which is, in fact, within a set range of a radio receiverwhich is monitoring the distance of the radio transmitter from the radioreceiver is falsely received as being out of range. This results in anerroneous condition of monitoring the distance of the radio transmitterfrom the radio receiver and further, may cause a panic situation orotherwise cause the person using the radio receiver to not trust thereliability of the radio tracking system.

An analysis of wireless prior art data transmission protocols inaccordance with accepted mathematical relationships for their evaluationreveals that they are poorly suited for data transmissions of more thana few characters in length. The following mathematical relationships areused to analyze fading: ##EQU1##

The threshold ST is the receiver threshold detection level and themedian SM is the median field strength level. ##EQU2##

The quantity t is the net probability of a fade divided by the mean rateof fading and equals

    1/2rF.sub.o (e.sup.+0.693r.spsp.2 -1)                      (5)

The fading rate F_(o) is the natural frequency at which atmosphericradio frequency transmissions periodically fade as a function of thechannel frequency F_(o) and the speed of the radio receiver in miles perhour; the fade length t in seconds is the length of fade; the fade belowthreshold F_(R) is the time duration in seconds that a transmissiondrops below the detection capability of the radio receiver; and theprobability of message loss P.sub.(error) is the probability that amessage transmission will not be completed as a result of a loss ofsynchronism between the data transmission and the receiver. See S. O.Rice; Statistical Properties of a Sine Wave Plus Random Noise; BellSystem Technical Journal, January, 1948; T. A. Freeburg; An AccurateSimulation of Multipath Fading; Paper; 1980; Caples, Massad, Minor; UHFChannel Simulator for Digital Mobile Radio; IEEE VT-29; May 1980; and P.Mabey, D. Ball; Application of CCIR Radio Paging Code No. 1; 35th IEEEV.T. Conf.; May 1985 for a discussion of the above-referenced equations.

U.S. Pat. No. 4,868,885 discloses the rapid measurement of a receivedsignal strength indicator (RSSI signal) generated from reception of areceived radio frequency signal which is used in a cellular radio systemto control handoff. Samples of the RSSI signal are taken successively intime and compared with the larger of the two samples being storedthroughout a desired sampling interval. Sample values exceeding thevalue obtained from an immediately preceding sample time and a valueobtained from an immediately succeeding sample time are stored twicewhile samples values that are less than an immediately preceding orsucceeding sample value are never stored. The resulting average is veryclose to a true average signal amplitude and is unaffected by Rayleighfading phenomena but is responsive to rapid changes in received signalamplitude caused by obstacles in the transmission path.

U.S. Pat. No. 5,193,216 detects when a radio receiver of the type whichreceives data transmissions is out of range. The radio receiver respondsto a decreasing slope of a RSSI signal after the receiver fails toreceive its coded identification code from the transmitter to signal theout of range condition. The '216 Patent discloses sampling the receivedsignal strength coincident with the detection of a predeterminedcharacteristic of the signal, such as the sync code, so that the signalfor which the received signal strength is measured is indeed the desiredsignal. If at the time the sync code is to be detected there is nosignal which is detected, a predetermined number of the most recentlystored RSSI values are read. If the slope of the stored RSSI valuesindicates that the radio receiver was moving toward an out of rangecondition before the loss of reception, a display is generated upon lossof reception indicating that the radio receiver is out of range from theradio transmitter.

DISCLOSURE OF INVENTION

The present invention is an improved radio tracking system comprised ofa mobile radio frequency receiver and at least one mobile radiofrequency transmitter. Each radio frequency transmitter periodicallybroadcasts a radio frequency carrier which is modulated with anidentification code which uniquely identifies the broadcasting radiofrequency transmitter which is decoded by the radio frequency receiver.The radio frequency receiver has an adjustable range control which setsa maximum range of movement of each radio frequency transmitter measuredfrom the radio frequency receiver that is permissible without thegeneration of an alert that a radio frequency transmitter has exceededthe set range. The range setting generates a voltage having a numericalvalue which is compared to a RSSI signal to determine if the set rangehas been exceeded. When the radio frequency receiver verifies that anidentification code transmitted with a modulated radio frequency carrieris assigned to a radio frequency transmitter which is being tracked ormonitored by the radio frequency receiver, the radio frequency receivergenerates the RSSI signal which is processed by a processor within theradio frequency receiver to compute an average of successively receivedRSSI signals from each of the radio frequency transmitters beingmonitored. The average is compared to the numerical value representingthe set range by the processor and the processor alerts the user of theradio frequency receiver when the set range for any receiver isexceeded.

Preferably, each RSSI signal is integrated to remove the effects ofelectrical noise before averaging. The average of RSSI signals andpreferably the average of the integrated RSSI signals generated fromtransmissions of the radio frequency carriers containing theidentification code of each radio frequency transmitter being monitoredand tracked are compared to the numerical value representing the setrange and an alert is generated by the microprocessor (preferably adigital signal processor) of radio frequency receiver when thecomparison reveals that at least one of the at least one radio frequencytransmitter is outside the set range.

Preferably, the average of the RSSI signals and the preferred average ofthe integrated RSSI signals is updated to include newly calculated RSSIsignals and preferably, newly calculated integrals of the RSSI signalsonly when each newly calculated RSSI signal or integral thereof differsfrom the computed average by less than a function of the average so asto exclude from the computation of the average those RSSI signals orintegrals thereof which differ from the average by more than thefunction. This process discards unreliable and statistically aberrantRSSI signals or integrals thereof which unreliable and statisticallyaberrant RSSI signals or integrals thereof would interject erroneousdata into the range determination process. Phenomena, such asinterference from people in the line of sight, Rayleigh fading,multipath interference, etc., can cause substantial magnitude variationof the magnitude of successively received RSSI signals or integralsthereof which falsely would be interpreted as motion of a radiofrequency transmitter outside the set range which is not occurring andwhich would cause an erroneous alert to be generated that a radiofrequency receiver has moved outside the range.

Once the radio frequency receiver determines that a radio frequencytransmitter has moved outside the set range, the user may switch theantenna configuration from an omnidirectional antenna to a directionalantenna by closing a "find me" switch in the housing of the radiofrequency receiver to permit directional tracking by the radio frequencyreceiver. Also, directional tracking may be performed by closing the"find me" switch any time the user of the radio frequency receiverdesires to monitor the position or motion of each radio frequencytransmitter being monitored.

A display of the magnitude of successive RSSI signals and preferably,integrals thereof, which are generated in response to the radiofrequency receiver detecting the radio frequency carrier containing theidentification code of the radio frequency receiver being tracked, isused to locate a direction from which a maximum signal magnitude of thesignal radio frequency carrier is being transmitted by the radiofrequency transmitter being tracked. The direction from which themaximum magnitude signal is being received, which is detected bydisplaying the magnitude of a quantity which is a function of individualRSSI signals generated by the reception of sequential transmissions ofthe identification code of the radio frequency transmitter beingtracked, is the true bearing of the radio frequency transmitter relativeto the radio frequency receiver. A preferred function without limitationis the integral or average signal magnitude of the RSSI signal which hasthe effects of noise removed.

The present invention further permits a user of each radio frequencytransmitter to close a "panic" switch to generate an alert which theuser of the radio frequency receiver responds to by closing the "findme" switch to cause the control processor to change the antennaconfiguration of the radio frequency receiver from an omnidirectionalantenna used for tracking all of the radio frequency receivers to adirectional antenna to permit directional tracking of the user of theradio frequency transmitter which transmitted the alert to the radiofrequency receiver. The directional tracking process by the radiofrequency receiver of each radio frequency transmitter transmitting analert is the same as the tracking function described above when a radiofrequency transmitter exceeds the set range.

The processor of the radio frequency receiver further utilizes errorcorrection code which is transmitted with the frames of informationencoding the identification code of each radio frequency transmitterwhich is being monitored or tracked to reconstruct valid data fromframes which cannot be corrected using the error correction code. In apreferred embodiment of the invention, an IDENTIFICATION FRAME GROUP,which is comprised of a plurality of frames with each frame containingbits of BCH error correction code and bits of many of the framesencoding the identification code of the radio frequency transmitter andone of the frame encoding the status of the user of the radio frequencytransmitter, is processed by the radio frequency receiver to determineif at least one erroneous uncorrectable bit is contained in any of theframes. Those frames containing at least one erroneous uncorrectablebit, which cannot be corrected by processing with the error correctioncode, are further processed to reconstruct valid data in the framecontaining the at least one erroneous uncorrectable bit by searching fora bit pattern of the erroneous uncorrectable bits being totally withinthe bits of the error correction code bit field. When the bits of theerror correction code of a frame totally contain the erroneousuncorrectable bits within the frame, the data which is theidentification code, status of the user of the radio frequencytransmitter or any other information may be recovered. The bit patternis a number of successive bits having an identical numerical value ofeither zero or one with the number being at least one greater than anumber of bits which may be corrected with the error correction code inthe frame which contains the at least one erroneous uncorrectable bit.As a result of reconstruction of frames by recovering valid data fromframes containing at least one erroneous uncorrectable bit, a greaternumber of radio frequency carriers containing the identification code ofthe radio frequency transmitters being monitored are detected. Thisenables the processing of a greater number of RSSI signals whichenhances the data which is processed to determine the range anddirection of the radio frequency transmitters being monitored asdescribed above.

In a preferred embodiment of the invention, the identification code ofeach of the radio frequency transmitters being monitored is encoded inframes containing error correction code. The bits of the frames modulatea subcarrier and the subcarrier modulating the radio frequency carrier.Analog modulation of the subcarrier or digital modulation of thesubcarrier may be used. The analog modulation modulates cycles of thesubcarrier with bits encoding the plurality of frames of theidentification code and any other information such as the information inthe IDENTIFICATION FRAME GROUP. Each cycle of the analog subcarrier ismodulated by bits at a plurality of separated angular positions. Digitalmodulation of the subcarrier modulates a pulse width of the subcarrier.The width of parts of the digital subcarrier are modulated with at leastone bit of the frames of the information. This form of subcarriermodulation permits the preferred form of data transmission as formattedinto the IDENTIFICATION FRAME GROUP to be rapidly transmitted at a lowerror rate which enhances battery life.

The processing of the detected individual cycles of the subcarrier bythe digital signal processor of the radio frequency receiver includescalculating an integral of at least one selected modulated part of eachof the individual cycles, numerically comparing each of the calculatedintegrals with a plurality of stored numerical ranges which ranges eachrepresent one of a plurality of possible numerical values that theselected part may encode to identify a stored range numericallyincluding the calculated integral and substituting for the at least oneselected part of each of the cycles the one of the plurality ofnumerical values representative of the identified stored range includingthe calculated integral with each numerical value encoding one bit whenthe subcarrier is an analog subcarrier and at least one bit when thesubcarrier is a digital subcarrier. Furthermore, the processing of thedetected individual cycles of the subcarrier by the digital signalprocessor includes calculating the integral by taking a plurality ofsamples of each selected modulated part of each of the individual cycleswith each sample having a numerical value and each sample is comparedwith a range of numerical values representing a valid sample whichshould be included within the calculation of the integral and when thecomparison reveals that the sample value is outside the range ofnumerical values, the compared sample value is replaced with a valuewhich is a function of the sample values adjacent the sample value whichis replaced. The compared sample value is preferably replaced with avalue which is an average of at least one sample value which precedesthe compared sample value and at least one sample value which exceedsthe compared sample value.

The above-described processes, which are performed by a digital signalprocessor of the radio frequency receiver for processing the modulatedcycles of the subcarrier, ensure that reliable detection of theidentification code of each radio frequency transmitter is achieved andreliable data which is a function of the RSSI signal generated duringthe reception of a valid identification code of one of the radiofrequency transmitter being monitored is used to determine the range anddirection of a radio frequency transmitter relative to the radiofrequency receiver. The reliability of the range detecting function andfurther the tracking function of each radio frequency transmitter uponthe generation of an alert by the radio frequency receiver when a radiofrequency transmitter moves out of range or further when a user of theradio frequency transmitter pushes the panic switch is directlyinfluenced by the reliability of the detection process of theidentification code of the radio frequency transmitter. The RSSIsignals, which are used ultimately to determine if a radio frequencytransmitter has moved outside the set range and further to track thedirection of a radio frequency transmitter relative to the radiofrequency receiver, are qualified by an accurate and high speeddetection of the identification code of each radio frequency carrierwhich is transmitted from each of the radio frequency transmitters beingmonitored. Therefore, a highly accurate detection process of theidentification code of each radio frequency transmitter by the radiofrequency receiver insures that the maximum number of qualified RSSIsignals are presented for further processing which enhances the accuracyof the determination if the range set by the user of the radio frequencyreceiver has been exceeded and further, the accuracy of the detection ofthe direction of the radio frequency transmitter relative to the radiofrequency receiver.

Furthermore, in accordance with the invention, the housing containingthe receiver has a display to permit the user of the receiver, who isdirectionally tracking at least one transmitter transmitting radiotransmissions to the receiver, to view the strength of the receivedradio transmissions to facilitate radio tracking. A field of viewlimiter is associated with the display and the housing to limit a fieldof view of the display of the strength of the radio transmissions towithin a field of view causing the user of the receiver to hold thereceiver at the waist or above and away from the body of the user tominimize radio interference with the transmissions in the line of sightbetween the at least one radio transmitter and the receiver. Preferably,the field of view limiter causes the user of the radio receiver to holdthe receiver away from the body and at or above chest level. The fieldof view defined by a pair of straight lines representing light raysrespectively extends from opposed edges of the display to correspondingopposed edges of an opening within the housing. The opening extendsinward from an outer surface of the housing to define a recess having abottom within the housing. The display is mounted on the bottom. Thefield of view subtends an angle which preferably is no greater than 45°and preferably 30° or less.

Moreover, a switch for activating the directional antenna of thereceiver is positioned relative to the housing so that the hand of theperson using the receiver unit to directionally track the at least onetransmitter holds the switch in a closed position with the directionalantenna being positioned relative to the housing so that during theholding of the switch in the closed position a line of sight between theantenna of the receiver and the at least one radio transmitter is notoccluded by the hand of the person holding the switch in the secondposition.

The aforementioned field of view limiter causes the user of the receiverunit to position it relative to the user's body to provide optimal radioreception of low power transmissions from the at least one transmitterbeing monitored. Reception of low power transmissions is important withthe present invention because of its preferred use of small batterys toprovide electrical power over many hours of continued use (e.g. 40 hoursor more). In this circumstance, the radiated power from the at least onetransmitter may be as low as five milliwatts which makes minimizing allforms of interference and positioning of the receiver in an optimalposition to provide maximum received signal strength extremely importantto achieve maximum distance of reception between the at least onetransmitter and the at least one receiver and maximum directionalsensitivity.

The positioning of the receiver in a position at or above the waist awayfrom the body of the user provides a spacing of one or more wavelengthsof the carrier of the transmissions which minimizes body interferenceand maximizes the height of the antenna of the receiver which alsoenhances signal reception. Moreover, positioning of the switch whichactivates the directional antenna relative to the housing of thereceiver which requires the hand of the user to close the switch whilethe hand is positioned out of the line of sight between the antennas ofthe at least one transmitter and the receiver also minimizesinterference caused by the user's hand.

Furthermore, the use of frequency hopping spread spectrum transmissionsby the receiver and the at least one transmitter permits acceptable andsufficiently accurate matching of identification code digits to qualifythe received signal strength indicator signal for further signalprocessing as described below without a complete match of stored andreceived identification code digits to achieve a reliable decoding ofthe identification code. Once a frequency hopping radio frequencyreceiver is synchronized to hop synchronously with the at least onefrequency hopping radio frequency transmitter being monitored for rangeand/or direction, a partial identification digit match between thetransmitted identification code digits and the receiver's storedcomplete transmitter identification code digits, which the synchronizedfrequency hopping radio frequency receiver is assigned to monitor,provides statistically reliable decoding sufficient to qualify thecorresponding received signal strength indicator signal for furtherprocessing which contributes to the generation of a highly reliableprocessed signal as described below used for range and/or directionaltracking. It is statistically improbably that a receiver will partiallydecode the identification code digits of a transmitter which is notsynchronously frequency hopping with the receiver.

A radio receiver unit for directionally tracking at least one radiotransmitter in accordance with the present invention includes a housingcontaining a radio receiver including a directional antenna forreceiving radio transmissions from the at least one radio transmitter; adisplay, which is electrically coupled to the receiver and fixed inposition with respect to the housing, for visually displaying a strengthof the radio transmissions received by the directional antenna; and afield of view limiter for limiting light emanating from the display to afield of view of the display when a user holds the receiver unit in theuser's hand away from the body of the user, the field of view beinglimited to planes extending upward from a plane of sight extending fromthe eyes of the user downward and intersecting a horizontal planeextending from the user's waist substantially at arms length of theuser. The field of view limiter limits the field of view of the displayby the user to planes extending upward from a plane of sight extendingfrom the eyes of the user downward and intersecting a horizontal planeextending from the user's chest substantially at arms length of theuser. The field of view limiter is contained within the housing. Thefield of view limiter comprises an opening within the housing extendinginward from an outer surface of the housing and having a bottom with thedisplay being positioned at the bottom. The field of view limited by thefield of view limiter is defined by a pair of straight linesrespectively extending from opposed edges of the display to opposededges of the opening.

A radio receiver unit in accordance with the invention further includesa switch, coupled to the receiver, the switch having a first position atwhich the directional antenna is not operative to receive the radiotransmissions from the at least one radio transmitter and a secondposition at which the directional antenna is operative to receive theradio transmissions from the at least one radio transmitter, the switchbeing positioned relative to the housing so that a hand of the user ofthe receiver during directional tracking of the at least one transmitterholds the switch in the second position; and wherein the directionalantenna is positioned relative to the housing so that during the holdingof the switch in the second position by the user's hand a line of sightbetween the directional antenna and the at least one radio transmitteris not occluded by the user's hand holding the switch in the secondposition. The field of view limiter is set in the housing in front ofthe directional antenna with reference to a line of sight extendingbetween the user and the at least one transmitter.

A radio receiver unit for directionally tracking at least one radiotransmitter in accordance with the invention includes a housingcontaining a radio receiver including a directional antenna forreceiving radio transmissions from the at least one radio transmitter; adisplay, which is electrically coupled to the receiver and fixed inposition with respect to the housing, for visually displaying a strengthof the radio transmissions received by the directional antenna from theat least one radio transmitter; and a field of view limiter for limitinglight emanating from the display to a field of view of the display by auser when holding the receiver unit in the user's hand away from thebody of the user and at least at waist height and above to an angle notgreater than 45° defined by a pair of straight lines respectivelyextending between opposed edges of the display and edges of the field ofview limiter. The angle is not greater than 30°. The field of viewlimiter is contained within the housing. The field of view limitercomprises an opening within the housing extending inward from an outersurface of the housing and having a bottom with the display beingpositioned at the bottom. The field of view limited by the field of viewlimiter is defined by a pair of straight lines respectively extendingfrom opposed edges of the display to opposed edges of the opening.

A radio receiver unit in accordance with the present invention includesa switch, coupled to the receiver, the switch having a first position atwhich the directional antenna is not operative to receive the radiotransmissions from the at least one radio transmitter and a secondposition at which the directional antenna is operative to receive theradio transmissions from the at least one radio transmitter, the switchbeing positioned relative to the housing so that a hand of the user ofthe receiver during directional tracking of the at least one transmitterholds the switch in the second position; and wherein the directionalantenna is positioned relative to the housing so that during the holdingof the switch in the second position by the user's hand a line of sightbetween the directional antenna and the at least one radio transmitteris not occluded by the user's hand holding the switch in the secondposition. The field of view limiter is set in the housing in front ofthe directional antenna with reference to a line of sight extendingbetween the user and the at least one transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system diagram of the present invention.

FIG. 2 illustrates the methodology of how the display of the radiofrequency receiver is used to locate the directional orientation of aradio frequency transmitter being tracked in accordance with the presentinvention.

FIG. 3 is a block diagram of a preferred embodiment of a radio frequencytransmitter in accordance with the present invention.

FIG. 4 is a circuit diagram of a preferred control CPU of the radiofrequency transmitter of FIG. 3.

FIG. 5 is a circuit diagram of a preferred power control and powersupply of the radio frequency transmitter of FIG. 3.

FIG. 6 is a circuit diagram of a preferred synthesizer/phase lock loopof the radio frequency transmitter of FIG. 3.

FIG. 7 is a circuit diagram of a preferred oscillator/modulator andpower divider of the radio frequency transmitter of FIG. 3.

FIG. 8 is a circuit diagram of a preferred power amplifier and antennaof the radio frequency transmitter of FIG. 3.

FIG. 9 illustrates a preferred protocol used for sending theidentification code of the radio frequency transmitter and the status ofthe user of the radio frequency transmitter to a radio frequencyreceiver in accordance with the present invention.

FIGS. 10A and 10B respectively illustrate analog and digital modulationof a subcarrier which is preferably used to encode the protocol of FIG.9.

FIG. 11 illustrates a constellation illustrating the analog modulationof the subcarrier of FIG. 10A.

FIG. 12 illustrates the digital modulation of the subcarrier of FIG. 10Bto encode groups of a plurality of bits in each half cycle of thesubcarrier.

FIG. 13 is a flowchart of the operation of the radio frequencytransmitter including the power on and initialization sequence.

FIG. 14 is a block diagram of a preferred embodiment of a radiofrequency receiver in accordance with the present invention.

FIG. 15 is a circuit diagram of a preferred control CPU of the radiofrequency receiver of FIG. 14.

FIG. 16 is a circuit diagram of a preferred power supply of the radiofrequency receiver of FIG. 14.

FIG. 17 is a circuit diagram of a preferred low noise amplifier, mixerand voltage controlled oscillator of the radio frequency receiver ofFIG. 14.

FIG. 18 is a circuit diagram of a preferred synthesizer/phase lock loopof the radio frequency receiver of FIG. 14.

FIG. 19 is a circuit diagram of a preferred second mixer, bandpassfilter and intermediate frequency amplifier and detector/demodulator ofthe radio frequency receiver of FIG. 14.

FIG. 20 is a circuit diagram of a preferred antenna reflector switch ofthe radio frequency receiver of FIG. 14.

FIGS. 21A and 21B illustrate the integration of the detected modulatedsinusoidal subcarrier in accordance with FIG. 10A by the digital signalprocessor of the radio frequency receiver of the present invention.

FIG. 22 illustrates the integration of the detected pulse widthmodulation subcarrier in accordance with FIG. 10B by the digital signalprocessor of the radio frequency receiver of the present invention.

FIGS. 23A and 23B illustrate sample processing performed by the digitalsignal processor of the radio frequency receiver of the presentinvention to remove noise transients in a pulse width modulatedsubcarrier in accordance with the present invention.

FIGS. 24A and 24B illustrate sample processing performed by the digitalsignal processor of the radio frequency receiver of the presentinvention to remove noise transients in a phase modulated sinusoidalsubcarrier in accordance with the present invention.

FIG. 25 is a flowchart of the operation of the digital signal processorof the radio frequency receiver of the present invention comparingintegrals of the detected sinusoidal or digital subcarriers withprestored ranges to convert the serial information modulated on thesubcarrier into a series of numerical representations of individual bitsor groups of bits which are modulated on the subcarrier in accordancewith the protocol of FIG. 9.

FIG. 26 illustrates a valid bit pattern of the frames in accordance withFIG. 9.

FIGS. 27-29 illustrate examples of bit patterns of frames in accordancewith FIG. 9 containing erroneous uncorrectable bits that are processedby the digital signal processor of the radio frequency receiver of thepresent invention to attempt to reconstruct valid data which cannot berecovered by processing the frames with only the error correction code.

FIG. 30 illustrates a block diagram of the operation of the radiofrequency receiver including the power on and initialization sequence.

FIG. 31 illustrates a waveform of a RSSI signal and its processingduring a single transmission interval of the identification code of aradio frequency transmitter being tracked by the radio frequencyreceiver of the present invention.

FIG. 32 illustrates the time variation of the individual integrated RSSIsamples and their average as a function of relative movement between theradio frequency transmitter and the radio frequency receiver.

FIG. 33 is a graph of free space loss in db as a function distancebetween the radio frequency receiver and the radio frequencytransmitter.

FIG. 34 is a graph of the RSSI voltage as a function of the receivedsignal level in dbm.

FIG. 35 is a table of free space loss as a function of separationdistance between a radio frequency transmitter and the radio frequencyreceiver.

FIG. 36 illustrates a range of positions of the radio receiver unit ofthe present invention relative to the body of a user to obtain optimalradio reception of transmissions in a line of sight with at least onetransmitter.

FIG. 37 illustrates optimal positioning of the receiver unit to obtainmaximum signal reception of the transmissions from the at least onetransmitter and optimal positioning of a user's hand relative to anon/off switch of the directional antenna and a line of sight between thedirectional antenna of the receiver unit and an antenna of the at leastone transmitter.

FIG. 38 illustrates a preferred embodiment of a field of view limiter ofthe display of the received signal strength of the transmissions fromthe at least one transmitter.

Like reference numerals identify like parts throughout the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates a system diagram of a radio tracking and rangingsystem 10 in accordance with the present invention. The system iscomprised of a radio receiver 12 and a variable number of radiofrequency transmitters 14-18. The designation "N" of radio frequencytransmitter 18 indicates that the total number of radio frequencytransmitters which could be monitored and tracked within the system 10may be any desired number. In a preferred embodiment of the inventiondescribed below, only two radio frequency transmitters are tracked by asingle radio frequency receiver. The detailed description of thearchitecture and operation of a preferred embodiment of the radiofrequency receiver 12 is described in conjunction with FIGS. 14-32 belowand a preferred embodiment of the radio frequency transmitters 14-18 isdescribed in conjunction with FIGS. 3-13 below. The radio tracking andranging system 10 has the capability, in the embodiment described below,with the radiated power of the radio frequency transmitters limited to100 milliwatts or less in accordance with power limiting regulations ofthe Federal Communications Commission for unlicensed wirelessapplications to detect radio frequency transmitters 14, 16 and 18 at arange which is calculated to be adjustable to possibly one mile measuredfrom the radio frequency receiver 12. The radio tracking and rangingsystem 10 further has the capability of tracking the direction of eachradio frequency transmitter 14, 16 and 18 relative to the radio receiver12 when either the radio frequency receiver determines that one or moreof the radio frequency transmitters has moved outside of a set distance20, which is variable by setting the range control 420 of the preferredembodiment 400 of the radio frequency receiver illustrated in FIG. 14 asdescribed below which causes the radio receiver to generate an alert, orwhen the user of one or more of the radio frequency transmitters 14, 16or 18 has generated an alert by pushing the panic switch 114 in thepreferred embodiment 100 of the radio frequency transmitter asillustrated in FIG. 3 as described below. Each radio frequencytransmitter 14, 16 and 18 is assigned an identification code whichuniquely identifies it from other radio frequency transmitters beingmonitored and tracked by the radio frequency receiver 12. Each radiofrequency transmitter 14, 16 and 18 periodically transmits its uniqueidentification code to the radio frequency receiver 12. Detection of theidentification code qualifies the RSSI signals used for determining ifthe set range 20 has been exceeded or for tracking the direction of theradio frequency transmitter relative to the radio frequency receiver asdiscussed below. The identification code may be periodicallytransmitted, such as ten times per second, to the radio frequencyreceiver 12. The radio frequency receiver 12 uses the decoding of theidentification code to qualify processing of each RSSI signalrepresenting the signal strength received at the antenna of the radiofrequency receiver of each radio frequency carrier which is detected andis determined to contain a valid identification code of one of the radiofrequency transmitters assigned to the radio frequency receiver todetermine the range and the direction of the radio frequencytransmitters relative to the radio receiver 12 as described below. Theidentification code is preferably encoded in at least one IDENTIFICATIONFRAME GROUP as discussed below in conjunction with FIG. 9. Each of theradio frequency transmitters 14, 16, and 18 preferably uses spreadspectrum frequency hopping of the radio frequency carriers. Each carrieris modulated with identification code of the transmitter, such that eachradio frequency transmitter repeatedly broadcasts its identificationcode on a cycling sequence of fifty frequencies. The frequency hoppingsequence of the radio frequency carrier is used to avoid interferencebetween other radio transmitters also using a radio frequency carrier toencode their identification codes with the IDENTIFICATION FRAME GROUP ofFIG. 9. Each of the radio frequency transmitters is preferablyprogrammed to have the same sequence of frequencies with thetransmissions of different transmitters being monitored at differenttimes by the radio frequency receiver. The probability of multiple radiofrequency transmitters synchronously hopping through the same sequenceof radio frequency carriers is so small that the probability ofinterference between the radio frequency carriers is small.

The radio frequency receiver 12 performs a sequence of signal processingoperations which substantially enhances the ability of the radiofrequency receiver to detect the identification code from each of themobile transmitters 14, 16 and 18 and further, upon detection of eachidentification code, processing operations which preferably includecalculation of an integral of the qualified RSSI signals, as describedbelow, to eliminate electrical noise therein and to further preferablycompute an average of successive integrations of the RSSI signal. Theaverage of the integrations of the RSSI signals accurately representsthe actual received signal strength to which a numerical valuerepresenting the set range 20 is compared to determine if any of theradio frequency transmitters 14, 16 and 18 are within or outside thevariable set range as illustrated in FIG. 1. Furthermore, in a preferredembodiment 400 of the radio frequency receiver 12, as described below,successive integrations of the RSSI signal are not used in thecalculation of the average of the integrations of the RSSI signal whenthey differ by more than a function of the average which, withoutlimitation, may be a percentage of the average of the integrated RSSIsignals such as twenty percent. Upon detection that any one of the radiofrequency transmitters 14, 16 and 18 is outside the set range 20 by adetermination that the average of the integrated RSSI values ofidentification code transmissions from each of the radio frequencytransmitters broadcasting a valid identification code is less than thevoltage representing the variable radius 20 produced by the rangecontrol 420 of FIG. 14, the radio frequency receiver 12 may be switchedby a user depressing the "find me" switch 426 of FIG. 14 to receivesubsequent radio frequency carriers containing a valid identificationcode from the radio frequency transmitter which is outside the set range20 with a directional antenna as described below. The radio frequencyreceiver 12 during the first portion of the monitoring operation inwhich it determines if any of the radio frequency transmitters 14, 16and 18 is outside of the set variable range 20 utilizes anomnidirectional antenna to receive radio frequency carriers containingthe valid identification codes which are transmitted from all of theradio frequency transmitters.

However, when the radio frequency receiver 12 determines that any of theradio frequency transmitters 14, 16 and 18 have moved outside of thevariable set range 20 or, alternatively, any of the users of the radiofrequency transmitters have indicated a change in their status bypushing the "panic" switch as described below, the radio frequencyreceiver is switched to receive the subsequent transmissions of theidentification codes with the radio frequency carrier with a directionalantenna. The magnitude of each individual RSSI signal which is qualifiedby reception of a valid identification code of the radio frequencytransmitter being tracked is displayed by the radio frequency receiver12 to provide information enabling the user of the radio frequencyreceiver to rotate the radio frequency receiver to an orientation whichproduces a maximum display of the successive integrated RSSI signals asdescribed below in conjunction with FIG. 2. The RSSI signal ispreferably integrated to remove the effects of noise as described below.The direction of the radio frequency transmitter 14, 16 or 18, relativeto the radio frequency receiver 12 from which the greatest magnitudeRSSI signals are sequentially generated by the reception of valididentification codes is the true bearing of the radio frequencyreceiver.

FIG. 2 illustrates how the display by the radio frequency receiver 12 ofthe magnitude of the integrated sequence of RSSI signals is used by theuser of the radio frequency receiver to track the direction of thetransmitter 16 which is outside the set range 20 of FIG. 1 by use of thedirectional antenna relative to the radio frequency receiver. Thedisplay of the magnitude of each integrated RSSI signal, which is notthe average of the integrated RSSI signals calculated during monitoringwith the omnidirectional antenna to determine if the set range 20 hasbeen exceeded, drives a magnitude indicator of the display which is aseries of lighted dots 24, such as those generated by LCDs or LEDs, todisplay the magnitude of each integrated RSSI signal produced inresponse to each reception of each valid identification code from theradio frequency transmitter being tracked. As illustrated in FIG. 2, themaximum number of dots 24 is activated in display 22 when the axis 26 ofthe directional antenna is directly pointed toward the radio frequencytransmitter 16 which is being tracked. While a maximum number of dots 24are illustrated as being activated in display 22, it should beunderstood that depending upon the distance of the radio frequencyreceiver 12 from the radio frequency transmitter 16, a lesser number ofthe dots would typically be activated. The displays 23 and 25, which aregenerated when the axis 26 is not directly pointed at the radiofrequency transmitter 16, have a lesser number of dots 24 activatedwhich is a function of the misalignment of the axis 26 of thedirectional antenna from direct alignment with the radio frequencytransmitter 16 as in display 22. It should be understood that therelative magnitude of the display of each successive integrated RSSIsignal will vary depending upon the alignment by the user of the radiofrequency receiver 12 of the axis 26 of the direction antenna toward theradio frequency transmitter 16 being tracked and/or relative motionoccurring between the radio frequency transmitter. The signal processingdescribed above and below eliminates the effects of interference andfading, etc., to minimize the display of erroneous magnitudes of theRSSI signals to provide highly accurate information useful for locatingthe direction of radio frequency transmitter 16 relative to the radiofrequency receiver 12. The display of the magnitude of each integratedRSSI signal, without the averaging used to determine when the set range20 is exceeded as explained above, permits motion of the radio frequencytransmitter 16 relative to the radio frequency receiver 12 to occurwithout an unacceptable time lag occurring in the display of the radiofrequency receiver representing the true direction of the radiofrequency transmitter relative to the radio frequency receiver.Furthermore, it should be understood that the illustration of thedisplay 22 showing a maximum number of the dots 24 activated when thereis true alignment of the directional antenna axis 26 with the radiofrequency transmitter 16 and a minimum number of the dots 24 beingactivated in display 23 when there is a misalignment by 90° of thedirectional antenna axis with the radio frequency transmitter is onlyintended for purposes of illustrating how direction finding isaccomplished. Namely, as the user of the radio frequency receiver 12rotates the axis of the directional antenna 26 toward true alignmentwith the radio frequency transmitter 16 from the positions representedby displays 23 and 25, an increasing number of the individual dots 24are activated in direct proportion to the magnitude of each integratedRSSI signal generated from each of the qualified successivetransmissions of the identification code of the radio frequencytransmitter which are received by the radio frequency receiver.

The radio frequency receiver 12 is designed to initially be clipped tothe belt of the person, such as an adult, tracking the position of twochildren. Furthermore, the radio frequency transmitters 14, 16 and 18may have a belt loop which prevents quick removal of the radio frequencyreceiver 12 from a child when, for example, an adult tries to defeat thetracking ability of the tracking system 10. Both the radio frequencyreceiver 12 and the radio frequency transmitters 14, 16 and 18 aredesigned to be powered with rechargeable batteries to provide up to apossible 40 or more hours of use between battery changes.

After the receipt of either a panic alarm, as generated by a user of theradio frequency transmitters 14, 16 and 18 caused by closing of the"panic" switch 114 of FIG. 3, or the detection by the radio frequencyreceiver 12 of the radio frequency transmitter being outside the setrange 20 by preferably averaging the integrated RSSI signals whilediscarding aberrant integrated RSSI signals from being included in theaverage of the integrals and comparing the average of the integratedRSSI signals to a set voltage representing the set range, the user ofthe radio frequency receiver 12 causes switching of the antenna of theradio frequency receiver from an omnidirectional antenna configurationused for tracking all of the radio frequency transmitters 14, 16 and 18to a true directional antenna having the axis 26 by closing the "findme" switch 426 of FIG. 14. After closing the "find me" switch, inaccordance with programming in the radio frequency receiver control CPU,which is preferably a digital signal processor, only a single one of theradio frequency transmitters is tracked, such as the radio frequencytransmitter 16 of FIG. 1 which has exceeded the set range 20.Alternatively, the invention may be practiced with the switching of theantenna configuration from an omnidirection to a directionalconfiguration under the control of the control CPU 106 of FIG. 3 withoutclosing the "find me" switch 426.

Tracking of only one radio frequency transmitter 14, 16 and 18 with theradio frequency receiver 12 at a time is desirable to avoid thepossibility of movement of the radio frequency receiver during trackingof one radio frequency transmitter causing another out of rangecondition to occur when the set range 20 is exceeded between the radiofrequency receiver and another radio frequency transmitter. This wouldthen create the undesirable circumstance of making it difficult to trackthe direction of the first radio frequency transmitter which, in thiscircumstance, is radio frequency transmitter 16 being outside the setrange 20.

The radio frequency receiver digital signal processor, as part of thepreferred process for averaging of the integrated RSSI signals, discardsany integration of a RSSI signal calculated from a single transmissionof an identification code from a radio frequency transmitter when thatintegrated value exceeds or is less than the average integrated value bya function of the average of the calculated integrals. This methodologyexcludes from the computation of the average of the calculated integralsnewly calculated integrals which differ from the average of thecalculated integrals by more than the function. The function may be aconstant, a percentage of the magnitude of the average of the calculatedintegrals, a scaler which varies in magnitude in accordance with themagnitude of the average of the RSSI signals or integrated RSSI signalsor any other mathematical expression which is designed to include onlythose integrated RSSI signals or non-integrated RSSI signals in thecomputation of the average used to determine if the set distance 20 hasbeen exceeded which represent valid signal strengths. This methodologyof discarding selected integrations of the RSSI signals or RSSI signalslessens the effects of Rayleigh fading and other fading phenomena frominfluencing the calculation of the average of the RSSI signals orintegrals thereof which can cause the average to fluctuate in a mannerwhich is not indicative of true distance of the radio frequencytransmitter 14, 16 or 18 from the radio frequency receiver 12 as isdiscussed below in conjunction with FIGS. 31 and 32. The thresholdamount of the function between the magnitude of the calculated averageof the integrated RSSI signals and a single new RSSI signal or integralthereof generated by the transmission of a single identification codefrom a transmitter to the receiver 12 may vary but it is believed thatan amount of 20% or less of the average is sufficient to insure thediscarding of unreliable and statistically aberrant integrations of theRSSI signal which are indicative of invalid range data.

The assumption is that because the range of the tracking capability ofthe system 10 is many hundreds of feet, a difference by an amount, suchas 20% between the average of the integrated RSSI signals or RSSIsignals used to compute the average and a single integrated RSSI signalor RSSI signal, would represent a physically impossible motion of theradio frequency transmitter 14, 16 or 18 relative to the radio frequencyreceiver 12 especially given the fact that the periodic broadcast of theidentification codes may be many times a second. In other words, if asmall child or an adult is being tracked, it would be physicallyimpossible for their motion to occur representing a significantpercentage of the maximum range 20 which may be tracked by the radiofrequency receiver 12 between successive samples. Furthermore, the setthreshold function between the average of the integrated RSSI signals orthe RSSI signals used to compute the average and the integrated value ofeach successive integrated RSSI signal or the RSSI signal may be lessthan 20% especially when the frequency of transmitting individualidentification codes from each of the radio frequency transmitters 14,16 and 18 to the radio frequency receiver 12 is at a relatively highfrequency, such as ten times per second, as described above.

FIG. 3 illustrates a block diagram of a preferred embodiment 100 of aradio frequency transmitter 12 in accordance with the present invention.The radio frequency transmitter 100 may be implemented with the circuitsillustrated in and described below in conjunction with FIGS. 4-8 inassociation with the source code of pages 1-7 of the Appendix. The radiofrequency transmitter 100 is designed to utilize 900 MHz. spreadspectrum technology which periodically transmits its identificationcode, as described above, preferably with utilization of the protocol,as described below, in conjunction with FIG. 9 and as generallydescribed in patent application Ser. No. 08/386,060, filed Feb. 7, 1995,entitled "System for Wireless Serial Transmission of EncodedInformation", U.S. patent application Ser. No. 08/385,312, filed Feb. 7,1995, entitled "Receiving Circuitry for Receiving Serially TransmittedEncoded Information", and U.S. patent application Ser. No. 08/385,143,filed Feb. 7, 1995, entitled "Transmitting Circuitry for SerialTransmission of Encoded Information".

The functional blocks of the radio frequency transmitter 100 illustratedin the block diagram of FIG. 3 may be implemented with commerciallyavailable integrated circuits as identified in FIG. 3 and in FIGS. 4-8.However, it should be understood that the invention may be practicedusing other circuits, including integrated circuits, than thoseillustrated in FIGS. 4-8. The main components of the radio frequencytransmitter are: oscillator/modulator 102, synthesizer/phase lock look104, control CPU 106, which is preferably a digital signal processor,power divider 107, loop filter 108, power amplifier 110, "panic" switch114, power control 115, batteries 117 which may be rechargeable andpower switch 119.

The oscillator/modulator 102 functions as a 900 MHz. oscillator whichincludes buffering electronics and functions as a modulator to encodethe identification information of the protocol as described below inconjunction with FIG. 9. FIG. 7 illustrates a preferred circuit forimplementing the function of the oscillator/modulator 102. The frequencyof oscillation of the oscillator/modulator 102 is determined by aninductor which, with parasitic capacitance that is present within theintegrated circuit board containing the transmitter, forms a tankcircuit which produces the rest frequency of the oscillator. The restfrequency is varied by variable magnitude DC voltage which is an inputof a pin of the integrated circuit of FIG. 7 from the control CPU 107.The DC voltage modulates the frequency of the oscillator/modulator 102to produce the sequential incrementing of the radio frequency carrierfrequency in a stair step fashion by the synthesizer/phase lock loop 104to sequentially change the frequency of the radio frequency carriermodulated with a subcarrier modulated with the IDENTIFICATION FRAMEGROUP of FIG. 9 to avoid interference with other transmitters. Theoscillator/modulator 102 produces the fifty different transmittingfrequencies which are used sequentially as the radio frequency carriersto broadcast successive IDENTIFICATION FRAME GROUPS of FIG. 9 containingthe transmitter identification code and the status of the "panic" switch114. The carrier frequency jumps approximately in a range between 100milliseconds and 400 milliseconds to a new transmitting frequency tobroadcast each successive IDENTIFICATION FRAME GROUP. A modulation inputpin of the integrated circuit of FIG. 7 provides the methodology forencoding the protocol as described below in conjunction with FIG. 9 tothe oscillator/modulator 102 from the control CPU 106. Multistagebuffers are provided within the oscillator/modulator 102 to preventloading of the oscillator/modulator sections and to provide anapproximate fifty ohm output impedance for direct coupling to the powerdivider 107 that immediately follows. A reference oscillator iscontained within the oscillator/modulator 102.

The synthesizer/phase lock loop 104 is a digitally programmable 900 MHz.synthesizer and phase lock loop circuit. FIG. 6 illustrates a preferredcircuit for implementing the function of the synthesizer/phase lock loop104. A prescaler is also contained within the synthesizer/phase lockloop 104 to take a sample of the oscillator frequency and compare it tothe preprogrammed frequency programmed by the control CPU 106 todetermine if any frequency error exists. Upon determination of anyfrequency error, a DC control voltage is varied and is sent through theloop filter 108 (to negate the effects of the modulation) to return therest frequency of the oscillator/modulator 102 to the desired frequency.The synthesizer/phase lock loop 104 is dynamically programmable to anyfrequency in the 902-928 MHz. band and is under direct digital controlof the control CPU 106. The synthesizer/phase lock loop 104, upon beingprogrammed by the control CPU 106, sends a DC control voltage,corresponding to the desired frequency of the fifty frequencies withinthe staircase of frequencies used to sequentially broadcast theIDENTIFICATION FRAME GROUP of FIG. 9, to the oscillator/modulator 102.As soon as the oscillator/modulator's frequency is sampled and comparedby the phase comparator with the desired frequency, a lock on frequencysignal is sent to the control CPU 106 to indicate that the radiofrequency transmitter is on the proper frequency and is prepared toreceive modulation information from the control CPU of theIDENTIFICATION FRAME GROUP of FIG. 9. The synthesizer/phase lock loop104 contains a master crystal oscillator. The reference frequency of themaster crystal oscillator is then utilized for comparison by the phaselock loop of the synthesizer/phase lock loop 104 to the preprogrammedfrequency to generate a control voltage to vary the frequency as needed.

The power divider 107 immediately following the oscillator/modulator 102is an integral part of a closed loop that determines the transmittingfrequency of the transmitter. FIG. 7 illustrates a preferred circuit forimplementing the function of the power divider 107. The power divider107 provides impedance matching and removes a portion of the power fromthe oscillator/modulator 102 for return to the synthesizer/phase lockloop 104 for sampling of the transmitted frequency. The power divider107 has discrete components that provide the correct impedance matchbetween the oscillator/modulator 102, the power amplifiers 110, asdescribed below, and an input to the prescaler of the synthesizer/phaselock loop. The power derived from the oscillator/modulator 102 bufferedoutput is a few milliwatts. An amount of this power (less than 50%) isremoved for frequency sampling by the synthesizer/phase lock loop 104.The remainder of the power obtained from the oscillator/modulator 102 isoutputted to the first stage PA1 of power amplifier 110.

The power amplifier 110 consists of two stages PA1, as referred toabove, and PA2, which amplify the output signal from the power divider107 to a power level of approximately 100 milliwatts. FIG. 8 illustratesa preferred circuit for implementing the power amplifier 110 and theantenna 112 which is a folded loop hybrid antenna. Each stage PA1 andPA2 of the two-stage power amplifier 110 has a fifty ohm input impedanceand output impedance which minimizes the number of coupling componentsrequired. The integrated circuit, which implements the power amplifier110, has a power control pin that permits the amplifiers to be placed ina deactivated state to conserve battery power when not in use.

The antenna 112 is made from a relatively heavy gauge wire and a portionof the printed circuit foil that provides the equivalent of a loadedfifty ohm quarter wave antenna. This type of antenna design provides anomnidirectional pattern that is affected minimally by circuit boardinfluences and has a high radiation efficiency. The antenna design issuch that it is broad band in its operation and therefore, will operateover a wide transmitting bandwidth as required for the frequency hoppingtechnique of spread spectrum technology utilized in the radio frequencytransmitter 100.

The control CPU 106 is preferably a digital signal processor. FIG. 4illustrates a preferred circuit for implementing the function of thecontrol CPU 106. The digital signal processor, which is used toimplement the control CPU 106, preferably includes a multitude offunctional components to provide the processing functionality requiredto provide the bits or groups of bits which encode the IDENTIFICATIONFRAME GROUP of FIG. 9 and to modulate the subcarrier with theIDENTIFICATION FRAME GROUP bits or groups of bits as described below inconjunction with FIGS. 10A, 10B, 11 and 12. The modulated subcarriermodulates each of the fifty radio frequency carriers. The modulatedradio frequency carriers provide transmit information which is used bythe radio frequency receiver 12 for determining the distance andlocation of the radio frequency transmitter 100 in mobile applicationssuch as finding children relative to the radio frequency receiver. Thedigital signal processor contains a high speed microprocessor, randomaccess memory, programmable read only memory, input/output ports,watchdog and reset electronics and all of the supervisory inputs tocontrol the functionality of the transmitter 100.

FIG. 4 illustrates a functional block diagram of the numerous controlfunctions which the digital signal processor performs to accomplish thetasks which the CPU 106 must perform. The digital signal processor hasstrap selectible inputs that determine the operating sequence of carrierfrequencies modulated with the subcarrier modulated with theIDENTIFICATION FRAME GROUP of FIG. 9 on which each radio frequencytransmitter of the plurality of radio frequency transmitters 14, 16 and18 will broadcast. Additional jumpers determine the uniqueidentification code of each radio frequency transmitter 14, 16 and 18that is utilized by the radio frequency receiver 12 as described belowto enable the radio frequency receiver to differentiate each of theradio frequency transmitters from which the radio frequency receiver 12may be receiving identification code transmissions as part of thetracking and ranging process. A test jumper is also included for initialfactory adjustment and servicing as required. The digital signalprocessor controls a piezoelectric transducer that alerts the user ofthe radio frequency transmitter via a series of beeps when the batteryvoltage is low indicating that the battery should be recharged, asdescribed below, in conjunction with FIG. 13. An input "panic" switch ofFIG. 4 permits the user of the radio frequency receiver 12 to performthe function of "panic" switch 114 of FIG. 3 that may be used by theuser of the radio frequency transmitter, as described above, to alertthe user of the radio frequency receiver 12, which would typically be anadult in the case of tracking children, that the user of the radiofrequency transmitter wishes to be found or requires assistance.

The digital signal processor also performs all of the necessarytransmitter power management functions to maximize the battery lifespanbetween recharging cycles. To accomplish this objective, the digitalsignal processor during periods of non-transmission, shuts down allunnecessary circuits to perform power conservation.

Additional data ports provide digital data control for thesynthesizer/phase lock loop 104 as described above which are necessaryfor programming of the desired and next desired radio carrier frequencyfrequencies when operation in a frequency hopping mode of spreadspectrum technology is used. The digital signal processor also has aninput data line that indicates status of the synthesizer/phase lock loop104. When a new operating frequency has been sent to thesynthesizer/phase lock loop 104, the digital signal processor waits fora lock on signal, as described below in conjunction with FIG. 13, via adata line to indicate that the synthesizer/phase lock loop hasprogrammed the oscillator/modulator 102 and that the oscillator thereinis on the correct operating frequency. Upon receipt of the lock onsignal, the digital signal processor continues to perform the necessarypowerup steps to prepare and send the protocol, as described below, inconjunction with FIG. 9 and FIG. 13.

The digital signal processor also has a logic input that permitsmonitoring of the status of the batteries. Upon change of logic level ofthe monitoring input, the digital signal processor will generate alerttones to indicate to the user of the radio frequency transmitter 100that the batteries are in need of recharging.

The digital signal processor also maximizes the battery lifespan byperforming numerous tasks which improve the operating efficiency of theradio frequency transmitter 100. Only those portions of the circuits ofthe radio frequency transmitter 100 which must be operational at anygiven time are turned on by the digital signal processor. For example,the digital signal processor, during its off duty cycle, remains in alow power consumption state and upon a predetermined timing cycle,commences the power up operation to permit the radio frequencytransmitter 100 to transmit. The digital signal processor first turns onthe power to the synthesizer/phase lock loop 104. The digital signalprocessor forwards via a serial data bus the desired frequency in theform of data to the synthesizer/phase lock loop 104. Immediatelyfollowing programming of the synthesizer/phase lock loop 104, thedigital signal processor turns on the power to the oscillator/modulator102. The digital signal processor then awaits a verification that theoscillator of the oscillator/modulator 102 has achieved the correctoperating frequency via the lock on signal from the synthesizer/phaselock loop 104. The digital signal processor then enables the poweramplifiers 110 and after a predetermined period of time, commencessending the digital data encoding the protocol, as described below inFIG. 9 and in detail FIG. 13, to the modulator of theoscillator/modulator 102. Upon completion of the transmission of theidentification code data of the radio frequency transmitter 100contained in the format of the IDENTIFICATION CODE FRAME of FIG. 9,discussed below, the digital signal processor begins an orderly shutdown of the power amplifier 110, oscillator of the oscillator/modulator102 and synthesizer of the synthesizer/phase lock loop 104.

At all times the digital signal processor monitors the "panic" switch ofFIG. 4 and the battery voltage. When the "panic" switch is pressed, thedigital signal processor immediately implements a powerup sequence (aspreviously described) and modifies the transmitted data within thecommand field CB of the IDENTIFICATION CODE FRAME, as described below inconjunction with FIG. 9, to update the panic status of the panic switch.

The power control 115 connects the batteries 117 through the powerswitch 119 to the various circuit components described above inconjunction with FIG. 3. FIG. 5 illustrates a preferred circuit forimplementing the function of the power control 115.

FIG. 9 illustrates an IDENTIFICATION FRAME GROUP which is an example ofa preferred serial protocol for encoding the identification code of theradio frequency transmitter 100, the command encoding the open or closedstatus of the "panic" switch 114 and other control information or datawhich is desired to be transmitted from the radio frequency transmitters14, 16 and 18 to the radio frequency receivers 112. The information istransmitted in time from left to right. The IDENTIFICATION FRAME GROUPtransmission is comprised preferably of six frames which are eachcomprised of forty-five bits. Each frame is comprised of twenty one bitsof error correction code which respectively is represented in labelledblocks of ten and eleven bits identified by the label "BCH". However, itshould be understood that the invention is not limited to the use of BCHerror correction code. Twenty one bits define the bit field of the errorcorrection code. The bits which are not contained in the errorcorrection code bit field are referred to as other bits and representdata to be processed after error code processing is completed with theerror correction code bits being discarded. The preceding three bitgroups of each frame contain groups of eight bits. The first two eightbit groups within the first three frames each contain a repeat of eightbits of identification information which uniquely identify the first twodigits of the transmitter identification code of the radio frequencytransmitter transmitting the IDENTIFICATION FRAME GROUP transmission.Each block labelled "I.D." contains two four bit nibbles respectivelyencoding the first two base ten digits of the transmitter uniqueidentification which, along with the other identification nibbleslabelled "three/four" in frame four collectively uniquely identify eachradio frequency transmitters transmitting the identification codeinformation and other information to the radio receiver 12. The threeeight bit groups, which respectively are contained in the first threeframes, contain a standard sync address S' which is repeated three timesas indicated to synchronize the clock of the radio frequency receivermicroprocessor to decode the IDENTIFICATION FRAME GROUP.

The S'/ID fields are binary serial data used by the radio frequencyreceiver 12 to detect the identification code and the command field CBwhich encodes the status of the "panic" switch 114. The digital signalprocessor of the radio frequency receiver 12, as described below, looksfor a bit pattern match that matches the preprogrammed synchronizationinformation S' and the ID digits of the identification code of thetransmitter. When a match occurs, the radio frequency receiver 12 turnson the balance of its electronics and begins the decoding process asdescribed below. After the repeat three times of a frame containing twodigits of identification code and the sync address S', the fourth frameof the ID frame group contains an eight bit command field CB which maycontain a command to the radio frequency receiver 12 that there has beena change in status of the user by closing the "panic" switch 114 of theradio frequency transmitter or another command(s) to specify otherfunctions to be performed by the radio frequency receiver. Theprogramming of the command field CB to reflect a change in status of the"panic" switch 114 is produced in response to the closing of the panicswitch 114 of FIG. 3. The fourth frame further includes four four-bitnibbles which encode identification digits three and four of theidentification code of the transmitter, which are contained in the nexttwo groups of eight bits after the command field CB followed by twogroups of ten and eleven bits making up the twenty-one bits of errorcorrection code as described above. The fifth frame contains three dataunits of eight bits which may be used for diverse functions such as thetransmission of additional information or commands from the radiofrequency transmitter 100 to the radio frequency receiver 12. The fifthframe also contains the BCH code as described above. Finally, the sixthframe contains two additional eight bit groups encoding data units fourand five each having eight bits which may contain data of the samegeneral function as described in conjunction with frame five. Finally,an end of frame marker EOF of eight bits is contained in the sixth framefollowed by the BCH error correction code as described above.

The bits encoding the IDENTIFICATION FRAME GROUP frame group of FIG. 9modulate a subcarrier as stated above which may be analog or digital.The modulated analog subcarrier may be a sinusoidal waveform asillustrated in FIG. 10A and the modulated digital subcarrier may be asquarewave as illustrated in FIG. 10B. Moreover, the number of bitsencoding the IDENTIFICATION FRAME GROUP of FIG. 9, which may modulateeach cycle of the subcarrier, may be varied from the four bits per cycleof FIG. 10A and the four bits per half of cycle of FIG. 10B. The highspeed integration capability of the digital signal processor used in theradio frequency receiver 12, as described below, consequent from highclock speed and a Harvard architecture permits multiples of the numberof bits encoded on each cycle illustrated in FIG. 10B and especially thesinusoidal subcarrier of FIG. 10A to be achieved with the invention. Themodulation of the subcarrier in either an analog or digital format withthe IDENTIFICATION FRAME GROUP provides a very high speed datathroughput of up to thirty-eight kilobaud which is significant in savingbattery power by reducing the time required to transmit theIDENTIFICATION FRAME GROUP which is an important consideration for theutility of tracking mobile radio frequency transmitters over a longperiod of time.

In FIG. 10A, the sinusoidal subcarrier is modulated at four differentphases (discrete angular positions) of a 360° cycle to encode a one or azero value of the individual bits of the IDENTIFICATION FRAME GROUP ofFIG. 9 or modifications thereof. As illustrated, the modulation isdiphase quadrature modulation (one or zero modulated at 45°, 135°, 225°and 315°). FIG. 11 illustrates a constellation representing the encodingof either a one or a zero at each of these four discrete angular phases.

In FIG. 10B a squarewave subcarrier is pulse width modulated with afirst half of the squarewave subcarrier cycle encoding four bits of thebits of the IDENTIFICATION FRAME GROUP of FIG. 9 or modificationsthereof. FIG. 12 illustrates possible numerical values representative offrame groups which may be encoded with squarewave modulation asillustrated in FIG. 10B. As illustrated, the pulse width modulation hassixteen possible widths encoding a four bit group which preferably areproportionate, i.e. a value of one is 1/16th the width of a value ofsixteen which facilitates high speed integration by the digital signalprocessor of the radio frequency receiver 12.

The analog or digital protocols of FIGS. 10A and 10B have the advantageof requiring less radiated power than other protocols, such as POCSAG orother digital protocols, such as ERMES or modifications thereof. Becauseof the application of the present invention for finding the wearer of amobile transmitter being limited to a maximum amount of radiated powerby the Federal Communications Commission of 100 milliwatts forunlicensed applications, the reduction in radiated power which isachieved with the use of the IDENTIFICATION FRAME GROUP transmission incombination with the processing capability of the digital signalprocessor of the radio frequency receiver 12 increases the effectiverange of the receiver's capability of tracking the mobile radiofrequency transmitters 14, 16 and 18.

FIG. 13 illustrates a detailed flowchart of the operation of the radiofrequency transmitter 100 of the present invention which has beengenerally described above in conjunction with FIGS. 3 and 4. Processingproceeds from the turning on of power at point 121 to step 123 where thecontrol CPU 106 is reset. Processing proceeds to point 125 where thepotential of the batteries 117 is read. Processing proceeds to decisionpoint 127 where a determination is made if the potential of therechargeable batteries 117 read at point 125 is too low to operate thetransmitter. If the answer is "yes" at decision point 127, processingproceeds to point 129 where the control CPU 106 causes warning beeps tobe emitted by the piezoelectric battery low indicator of FIG. 4 to alertthe user of the low battery condition. If the answer is "no" at decisionpoint 127, processing proceeds to point 129 where a check is made forthe identification code and the frequency inputs for determining theoperation parameters of the transmitter, including its frequency hoppingsequence, which is used to avoid interference with other radio frequencytransmitters. The processing proceeds to point 131 where the firstfrequency of the frequency hopping sequence is programmed. Theprocessing proceeds to point 133 where the oscillator/modulator 102 isturned on. The processing proceeds to point 135 where a wait interval ofa set number of milliseconds is entered into to permit the poweramplifier 110 to become operational prior to proceeding to decisionpoint 137 where a determination is made of whether or not the frequencyof the oscillator is locked on to the frequency commanded by the controlCPU 106. If the answer is "yes" at decision point 137, processingproceeds to point 139 where the power amplifier 110 is turned on. Theprocessing proceeds to point 141 where another delay of a specifiednumber of milliseconds is entered into to permit the power amplifier 110to become operational. Thereafter, at point 143, the subcarrier ismodulated with the IDENTIFICATION FRAME GROUP of FIG. 9 including theidentification code of the radio frequency transmitter and the storedstatus of the command field CB reflecting the previous state of theclosing of the "panic" switch 114. At this point, the memory of thecontrol CPU 106 stores a digitized version of the modulated subcarrierin either analog format of FIG. 10A or digital format of FIG. 10B toencode the IDENTIFICATION FRAME GROUP. The processing proceeds todecision point 145 where the control CPU 106 again determines if thepotential of the battery 117 is low. If the answer is "yes" at decisionpoint 145, processing proceeds to point 147 where warning beeps arecaused to be emitted under control of the control CPU 106 which areanalogous to the beeps emitted at step 129 as described above.Processing proceeds from decision point 145 if the answer is "no" andfrom point 147 to point 149 where the power shutdown sequence isperformed. Processing proceeds to decision point 151' where adetermination is made if the "panic" switch 114 has been closed. If theanswer is "yes" at decision point 151', processing proceeds to point153' where the status of the user is changed in memory of the controlCPU 106 to cause the command field CB of FIG. 9 as described above to bechanged to alert the radio frequency receiver 12 of the change in statusof the "panic" switch 114 which will be transmitted with the next radiofrequency carrier. Processing proceeds from the change in status code atpoint 153' or if the answer at decision point 151' is "no" to point 155'where the next frequency of the frequency hopping sequence of the radiofrequency carrier is selected. Processing proceeds from point 155' backto point 133 where the oscillator/modulator 102 is turned on asdescribed above. If the answer at decision point 137 is "no" that theradio frequency transmitter is not locked on to the commanded frequencyof the radio frequency carrier, processing proceeds to decision point157' where a determination is made if the battery 117 is at a lowpotential. If the answer is "yes" at decision point 157', the processingproceeds to point 159' where warning beeps are emitted which areanalogous to points 147 and 129 as described above. If the answer is"no" at decision point 157' that the battery is not low, or warningbeeps have been emitted at step 159', processing proceeds to decisionpoint 161' where a determination is made if a timer has expiredindicating that the radio frequency transmitter has not locked onto theprogrammed frequency within a predetermined period of time. If theanswer is "yes" at decision point 161', processing proceeds to point163' where warning beeps are emitted which are analogous to the warningbeeps emitted at steps 159', 147, and 129 described above. Processingproceeds from point 163' to the end of service. If the answer is "no" atdecision point 161', processing proceeds back to decision point 137 asdescribed above.

FIG. 14 illustrates a block diagram of a preferred embodiment 400 of theradio frequency receiver 12 of FIG. 1. The embodiment 400 functions as a900 MHz. spread spectrum radio frequency receiver that is capable ofreceiving and monitoring the transmissions of the radio frequencytransmitters 14, 16 and 18 described above which contain the informationpreferably of the format of FIG. 9. The embodiment 400 functions toaccurately analyze the identification code status of the "panic" switch114 and other information. Upon determining that the IDENTIFICATIONFRAME GROUP of FIG. 9 contains an identification code of a radiofrequency transmitter assigned to the radio frequency receiver 400 formonitoring and tracking purposes, the embodiment 400 determines thedistance of the radio frequency transmitter from the radio frequencyreceiver as well as the bearing of the radio frequency transmitterrelative to the radio frequency receiver when the antenna configurationof the radio frequency receiver is switched from an omnidirectionalpattern which is used to monitor the group of radio frequencytransmitters 14, 16 and 18 to a directional antenna which is used tomonitor the range and direction of a single radio frequency transmitter.The embodiment 400 utilizes highly integrated commercially availableintegrated circuits to provide a small, compact, battery operated radiofrequency receiver which may be carried by the operator thereof on abelt loop or otherwise on or with the person.

The main components of the embodiment 400 of the radio frequencyreceiver are as follows: Control CPU 402 which is preferably a digitalsignal processor, a synthesizer and phase lock loop 404, antenna array405, antenna reflector switch 406, low noise amplifier 407, first mixer408, first intermediate bandpass filter 409, local oscillator 410, localoscillator 412, second mixer 414, second bandpass filter andintermediate frequency amplifier 416, data detector/demodulator 418,range setting control 420, analog to digital converter 422, display 424,"find me" switch 426 and alerting device 428.

The control CPU 402 which, as stated above, is preferably a digitalsignal processor, is illustrated in FIG. 15 and provides the control ofthe embodiment 400 which permits the determination of range of themultiple radio frequency transmitters 14, 16 and 18 relative to the setrange limit 20 specified by the setting of potentiometer 420 andfurther, the determination of the direction of a radio frequencytransmitter relative to the radio frequency receiver, as describedabove, in conjunction with FIG. 2 either when a radio frequencytransmitter has moved beyond the set range 20 or has instituted a "findme" command by closing of the "find me" switch 426. FIG. 15 illustratesa preferred circuit for implementing the control CPU 402.

The digital signal processor contains three eight-bit I/O ports that areutilized for the various control and data functions, 6K of ROM memorythat contains the operating program, and 176 Kbytes of RAM memory. Thedigital signal processor also contains an eight-bit analog to digitalconverter which corresponds to the analog to digital converter 422 withan eight input multiplexer, reset and initialization watch dogs, aserial port, programmable timers, and the master processor oscillator.The digital signal processor controls via the digital ports thereceiving frequency (frequency control lines) and the mode of theantenna array 405 (directional or omnidirectional control). The digitalsignal processor also drives light dots (illustrated in FIG. 2 a dots24) of the LCD or LED display 424 which indicate a power on status andfurther the amplitude of the RSSI signal which, as described above inconjunction with FIG. 2 is preferably integrated, to eliminate theeffects of noise. Additional lines are utilized to drive thepiezoelectric alert speaker 428 which provides a warning to the user ofthe embodiment 400 that one or more of the radio frequency transmitters14, 16 and 18 has moved out of the set range 20 or that the "panic"switch 114 of the radio frequency transmitter of FIG. 3 has been closedto signal the user of the radio frequency receiver that a user of one ofthe radio frequency transmitters 14, 16 and 18 wishes to be found or isan emergency situation, etc.

Inputs to the digital signal processor are accomplished via the dataports. The closing of the "find me" switch 426 causes the digital signalprocessor to change the configuration of the antenna array 405 by acommand from the control CPU 402 to change the antenna reflector switch406 to change the antenna array to be configured in a directional arraysuch that the user of the radio frequency receiver 400 can attempt toline up the axis 26 of the directional antenna in the direction where amaximum magnitude RSSI signal is displayed on the dots 24 as describedabove in conjunction with FIG. 2. Furthermore, the detecting of thechange in status of the "find me" switch 426 by the digital signalprocessor causes the digital signal processor to be conditioned forprocessing other necessary functions. The demodulated data which isreceived from detector/demodulator 420 is sent to the digital signalprocessor via data lines.

The analog to digital converter 422 performs a multitude ofdigitizations of sensed or inputted analog signals. One input is usedfor the measurement and monitoring of the battery condition. The analogto digital converter 422 digitizes the measured battery voltage forcomparison to a stored operating voltage in the memory of the digitalsignal processor. When the monitored battery voltage falls below thepredetermined threshold, the digital signal processor initiates a lowbattery warning.

A second input to the analog to digital converter 422 is connected tothe analog RSSI signal which is outputted from the intermediatefrequency amplifier within the bandpass filter/intermediate frequencyamplifier 416 which is digitized for further processing including thepreferred integration thereof to remove the effects of noise, thecomputing of averages of RSSI signals received from each of the radiofrequency transmitters 14, 16 and 18 and the discarding of aberrantintegrations for each RSSI signal integral which differ by the functionas described herein. Up to hundreds of samples of the RSSI signal aremade of each RSSI signal which is received to remove the effects ofelectrical noise as described. The samples are then further processed toprovide a highly filtered and accurate distance measurement by theaveraging process and the discarding of aberrant integrations asdescribed.

A third input to the analog to digital converter 422 measures the DCvoltage produced by the range setting of the range control 420 that ispreset by the user of the embodiment 400. The measured DC voltage fromthe range control 420 is proportional to the desired range 20 andprovides a comparison voltage necessary to determine when the set rangehas been exceeded. The preset range control voltage produced by therange control 420 is compared to the average of the RSSI signals whichare preferably integrated prior to averaging to remove the effects ofnoise to perform the alerting function that one or more of the radiofrequency transmitters 14, 16 and 18 has exceeded the set range.

The control processor portion of the digital signal processor providesall of the processing necessary to perform the decoding of thesubcarrier as modulated with the IDENTIFICATION FRAME GROUP, asdescribed above in conjunction with FIG. 9 and below in conjunction withFIGS. 21A, B, 22, 23A, B, 24A, B and 25, and operational status. Thecontrol processor portion of the digital signal processor also performsthe necessary averaging of the RSSI signals generated in response to thereception of a valid identification code from each of the radiofrequency transmitters 14, 16 and 18 which, as stated above, preferably,is an average computed from integrated RSSI signals to remove theeffects of noise to provide an accurate determination of the range ofthe radio frequency transmitters 14, 16, 18 from the radio frequencyreceiver 12.

The digital signal processor also provides power management of theembodiment 400 to maximize the operating life of the battery. Only theportions of the embodiment 400 that need to be operational at any giventime are turned on by the digital signal processor. For example, thedigital signal processor during its off duty cycle remains in a lowpower consumption state and upon a predetermined timing cycle, commencesthe power up operation. To prepare the embodiment 400 to receive theradio frequency carrier containing the IDENTIFICATION FRAME GROUP, thedigital signal processor first turns on the power to thesynthesizer/phase lock loop 404. The digital signal processor thenforwards via the serial data bus the desired frequency control to thesynthesizer/phase lock loop 404. Immediately following the programmingof the synthesizer/phase lock loop 404, the digital signal processorturns on the power of the voltage controlled oscillator 410 associatedwith the first mixer 408. The digital signal processor then awaitsverification that the voltage controlled oscillator 410 has achieved theoperating frequency via the lock on signal from the synthesizer/phaselock loop 404. The digital signal processor then simultaneously monitorsthe output of the detector/demodulator 418 for data being received inthe format of the IDENTIFICATION FRAME GROUP of FIG. 9 and performsdigital monitoring of the RSSI signal which is outputted by theintermediate frequency amplifier of the intermediate frequency amplifierand bandpass filter 416. This process continues until the transmittedradio frequency carrier is received in its entirety at which time thedigital signal processor begins an orderly shut down process.

At all times, the digital signal processor is monitoring the batteryvoltage, as well as the "find me" switch 426. When the "find me" switch426 is depressed by the user of the embodiment 400, the digital signalprocessor immediately implements the power up sequence (as previouslydescribed) and modifies the control program to display the integratedRSSI signal on the dots of the display 424.

The synthesizer/phase lock loop 404 is a digitally programmable 900 MHz.synthesizer and phase lock loop circuit. FIG. 18 illustrates a preferredcircuit for implementing the synthesizer/phase lock loop 404. Thesynthesizer/phase lock loop 404 also contains a prescaler to permitsampling of the oscillator frequency for comparison to the commandedfrequency which is specified by the control CPU 402 to determine if thefrequency is correct. The synthesizer/phase lock loop 404 receivesdigital data from the control CPU 402 that determines the desiredoperating frequency. The synthesizer/phase lock loop 404 then translatesthe received digital frequency information into an analog voltage thatis applied to the voltage control oscillator.

The synthesizer/phase lock loop 404 is capable of operating at thousandsof different frequencies in the 902-928 MHz. band and is programmable toa subset of fifty frequencies by the control CPU 402 which frequenciescorrespond to the frequencies which are programmed to be used by theradio frequency transmitters 14, 16, and 18.

An integral part of the synthesizer/phase lock loop 404 is a masterreference oscillator that provides a high stability reference frequencythat is utilized to generate the desired 900 MHz. receiving frequencythat is applied to the mixer 408 to shift the received radio frequencycarrier down to a first intermediate frequency.

The low noise amplifier 407 has two stages and is directly coupled tothe receiving antenna array 405 at is input through the antenna switch408 and to the mixer 408 at its output. The low noise amplifier 407 iselectronically controlled by the control CPU 402 to permit maximumbattery savings when the embodiment 400 is not active. The low noiseamplifier 407 provides approximately 11.5 dB of gain ±0.2 dB over theentire 902-928 MHz. operating band.

The first mixer 408 is connected to voltage controlled oscillator 410that is tuned by external coils and capacitors and a varactor diode topermit the oscillator frequency to be controlled directly by thesynthesizer/phase lock loop 404. FIG. 17 illustrates a preferred circuitfor implementing the low noise amplifier 407, first mixer 408 andvoltage controlled oscillator 410. The analog voltage generated by thesynthesizer/phase lock loop 404 is coupled to a varactor diode of FIG.17 which changes the resident frequency and hence the operatingfrequency of the voltage controlled oscillator 410 to the desiredfrequency. The oscillator of FIG. 17 has a frequency monitoring pin thatprovides a feedback signal to the synthesizer/phase lock loop 404prescaler. This provides a closed frequency monitoring loop that permitsthe synthesizer/phase lock loop 404 to compare frequency of the voltagecontrolled oscillator 410 to the desired frequency requested by thecontrol CPU 402. When the desired frequency and the operating frequencyof the voltage controlled oscillator 410 differ, an error voltage isgenerated that changes the frequency of the voltage controlledoscillator to provide the correct frequency. The DC control voltage isfiltered by components R6 and C46 of FIG. 17 to prevent oscillatorinstability.

The first mixer 408 is also contained within the circuit of FIG. 17which mixes the oscillator output with the incoming filtered radiofrequency signal outputted by a low noise amplifier 407 to produce theintermediate operating frequency. This intermediate frequency is aproduct of the two frequencies being mixed together. The resultantfrequency and related undesired mixer frequencies are transmitted to thefirst intermediate frequency bandpass filter 409.

The bandpass filter 409 is comprised of discrete components that permitonly the desired band of RF frequencies to pass from the first mixer 410to the second mixer 414 and is the first of a series of bandpassfilters. The first intermediate frequency bandpass filter 409 consistsof a two-stage crystal lattice filter that is tuned to 10.7 MHz. Thefirst mixer 408 produces this frequency as well as several undesiredfrequency components that are filtered out by the first intermediatefrequency bandpass filter 409. When the two frequencies are mixed, e.g.900 MHz. and 910.7 MHz., several mixed frequencies result. The first isthe frequency that is the sum of the two frequencies and another is thedifference. The embodiment 400 uses the difference frequency of 10.7MHz. with the first intermediate frequency bandpass filter 409 passingonly that frequency and not the other undesired frequency. The output ofthe first intermediate frequency bandpass filter 409 is applied to animpedance matching network (not illustrated in FIG. 14) which is coupledto additional gain stages in the second mixer 414.

The second mixer 414 is part of a double conversion receiver designwhich provides the highest sensitivity and greatest rejection ofadjacent channel interference and unwanted signals. FIG. 19 illustratesa preferred circuit for implementing the local oscillator 412, secondmixer 414, second bandpass filter/intermediate frequency amplifier 416and detector/demodulator 418. The output from the matching network andthe output of local oscillator 412 are applied to the second mixer 414to convert the signal down to a second lower intermediate frequency of455 KHz. The second mixer 414 is similar to the first mixer 408 in thatit produces signal components that must be filtered by the secondintermediate frequency bandpass filter 416 and intermediate frequencyamplifier so that only the desired intermediate frequency is applied tothe demodulator/detector 418. The second intermediate frequency bandpassfilter of the second intermediate frequency bandpass filter andintermediate frequency amplifier 416 is a two-stage filter. Theamplification produced by the intermediate frequency amplifier isproduced by multiple stages to provide the necessary amplification forappropriate signal detection by the detector/demodulator 418.

The detector/demodulator 418 receives the amplified intermediatefrequency signal from a second limiting amplifier portion of theintermediate frequency amplifier section of the second bandpass filterand intermediate frequency amplifier 416 which is applied to a Gilbertcell quadrature detector. One port of the Gilbert cell is internallydriven by the intermediate frequency amplifier. The second output of thepreceding intermediate frequency amplifier is AC coupled to a tunedquadrature network. This signal, which now has a 90° phase relationshipto the internal signal, drives the other port of the multiplier cell.The demodulated output of the quadrature detector drives an internaloperational amplifier. This operational amplifier provides additionalgain of the recovered and detected signal containing the information ofthe IDENTIFICATION FRAME GROUP of FIG. 9 and a minimal amount offiltering prior to coupling of the demodulated data of theIDENTIFICATION FRAME GROUP to the control processor 402.

The intermediate frequency amplifier section of the second bandpassfilter and intermediate frequency amplifier 416 provides the RSSI signalwhich is processed as described above and below. The RSSI signal voltageis proportional in scale to the field strength of the radio frequencycarrier received by the antenna array 405. When interferences occur,such as Rayleigh fading and multipath signals as described below inconjunction with FIGS. 31 and 32, the RSSI signal varies dramaticallywhen observed on an instantaneous basis. The digital signal processor isutilized to provide a series of calculations of integrals or averages ofthe RSSI signals as described to remove the unwanted and instantaneousvariations that typically render the RSSI signal useless or unreliablefor range measurements. The RSSI signal is forwarded to the analog todigital converter 422 as described above. A detailed explanation of theintegrations or averaging of the RSSI signals is described below inconjunction with FIGS. 31 and 32.

The electronic antenna switch 406 is controlled by the control CPU 402.In the normal omnidirection receiving mode the radio frequency receiverantenna switch connects the reflector element 432 of the antenna array405 to the driven element 431 of the antenna to produce anomnidirectional configuration. A preferred circuit for implementing theantenna switch 406 is illustrated in FIG. 20. The use of the antennareflector switch 406 to switch the configuration of the antenna assembly405 between omnidirectional and directional modes is important inreducing the size and number of antenna components in a small formfactor for wearing on a person's belt to obtain acceptable antennaperformance for achieving the two different and competing signalreception characteristics for omnidirectional and directional reception.In this configuration, the received signal pattern is omnidirectionaland the reflector assembly 432, as well as the driven portion of theantenna array 405, are coupled together to receive signals from theradio frequency transmitters 14, 16 and 18 by opening the antenna switch406 to receive an omnidirectional pattern of the received radiofrequency carriers and to couple them to the low noise amplifier circuit407.

When the user closes the "find me" switch 406, which is also illustratedin FIG. 15, the control CPU 402 sends a digital signal to the antennareflector switch 406 that changes the antenna configuration to a smallaperture reflective array having the antenna axis 26 as described abovein conjunction with FIG. 2. This is accomplished by grounding of thereflector array 432 by closing the antenna reflector switch 406. In thismode, only the center driven antenna portion 431 is connected to the lownoise radio frequency amplifier 407 to provide highly directionalreception of the radio frequency carriers to permit the user todetermine the direction from where signals are received produces themaximum magnitude of the integrated RSSI signal relative to thealignment of the axis 26 of the directional antenna array 405. The axis26 may be thought of as a pointer toward the mobile radio frequencytransmitter 14, 16 or 18 being tracked. As explained above, when theaxis 26 is pointing directly at the radio frequency transmitter 14, 16or 18 whose range and position is being monitored, a maximum number ofthe dots 24 is activated as illustrated in FIG. 2. As has been explainedabove in conjunction with FIG. 2, the user rotates the radio frequencyreceiver 12 until a maximum number of the dots 24 is activated whichsignals the true direction of the radio frequency transmitter relativeto the radio frequency receiver 12.

The antenna array 405 consists of two active components. The first isthe driven or center element 431 which is composed of a heavy gauge wirethat is matched to the low noise RF amplifier 407 by discretecomponents. The second element of the antenna array 405 is the reflectorassembly 432. The reflector assembly 432 surrounds the driven element ina somewhat cylindrical fashion with a slot facing outward which is theantenna axis 26 and extends away from the user of the embodiment 400.The slot (not illustrated) permits the radio frequency carriertransmitted from the radio frequency transmitter being tracked, whichhas been modulated with the subcarrier modulated with the IDENTIFICATIONFRAME GROUP information, to enter the reflector assembly 432 of theantenna to be received by the driven element 431.

As stated above, in the omnidirectional mode, the antenna switch 406connects the reflector array 432 to the driven element 431 tocollectively combine the two elements into a single receiving antenna.In this configuration, the antenna is omnidirectional and the reflectiveelement 432 and the driven element 31 collectively contribute toreceiving the radio frequency carrier containing the IDENTIFICATIONFRAME GROUP information.

When the embodiment 400 is changed to the directional mode by switchingthe antenna reflector switch 406 under control of the control CPU 402,the antenna reflector switch disconnects the reflector element 432 andconnects it to ground. Only the driven element 431 is used to receivethe signal and the array becomes highly directional to the surroundingof the driven element by the reflector assembly 432. As is explainedabove, preferably the signal after qualification by the control CPU 402that a valid identification code has been received is integrated byintegrating the output of the RSSI signal from intermediate frequencyamplifier of the second bandpass filter and intermediate frequencyamplifier 416.

The integration of an analog subcarrier modulated, as illustrated inFIGS. 10A and 12, as part of the demodulation process is explained indetail as follows. FIG. 21A illustrates the received diphase quadraturemodulated subcarrier as received from the detector/demodulate 418 of theradio frequency receiver. The data, modulates the subcarrier at the 45°and 135° phases with the 225° and 315° phases having been omitted fromthe illustration. Regardless of the number of spaced apart angularpositions of the subcarrier which are modulated, the determination ofwhether a one or a zero is encoded in the modulation involves thediscrimination of whether the integral falls on the "one" or "zero" sideof the boundary on the vertical voltage axis V representing themagnitude of the integral. The lower magnitude voltage V range along theY axis represents the encoding of a binary zero at 45° and the highermagnitude voltage range represents the encoding of a binary one at 135°.

The embodiment 400 has a digital signal processor clock which issynchronized by ID/S' field of FIG. 9 to the frames of the incomingIDENTIFICATION FRAME GROUP. This synchronization permits the digitalsignal processor to integrate in a window around the exact angular phaseof where the modulation of each bit is placed. The sampling of thevoltage, may begin at 35° and end at 55°. In the 20° window, the digitalsignal processor computes hundreds of samples which are integrated. Thesize of the window and the number of angular positions of the subcarrierwhich are modulated may vary in practicing the invention with muchhigher numbers of bits modulated per quadrant of the subcarrier beingpossible than illustrated in FIG. 10A.

FIG. 21B illustrates a simplified example of computing the integral ofthe waveform at 45° in FIG. 21A where only eleven samples are takenwhich have an integrated value of eight. Once the integrated value isobtained, the digital signal processor looks in a prestored lookup tableas described below in detail in conjunction with FIG. 25 which permits avalue of zero to be within a numerical integration range between zeroand sixteen. In FIG. 21A it can be that the numeric value for the datacontained at the 135° phase will be greater than sixteen. Therefore, thesame integration process and comparison with the range of prestoredvalues centered in a 20° window around 135° yields a value of one at the135° phase.

The actual values obtained in each step of the integration process willtypically be much higher than the foregoing example of FIGS. 21A and B.The actual values obtained in each step of the integration process willbe dependent upon many variables determined primarily by the receivingcircuitry. The operating voltage, A to D sampling speed, and clock speedof the digital signal processor will all influence the actual numericvalues obtained in this integration process. However, the transmittedwaveform will appear essentially the same for all mobile data productsusing the invention. Each of the different received data waveforms willhave different binary values and different binary ranges in their lookuptables.

The integration of a squarewave subcarrier with each half being pulsewidth modulated with four bits (numerical widths varying between one andsixteen), as illustrated in FIGS. 10B and 12 as part of the demodulationprocess, is described as follows with reference to FIG. 22. In thissimplified example, the digital signal processor takes ten samples ofthe detected subcarrier where in actual practice hundreds of sampleswould be taken. The previously stored sample values representing thewaveform are processed by the digital signal processor to integrate thearea under the waveform. In actual practice, the number of samples willbe dependent upon the sampling speed of the A to D converter 422 and theclock speed of the digital signal processor. In this example, there is afixed numerical value assigned to the X axis and a value that isrepresentative of the received voltage V of the waveform on the Y axis.The digital signal processor uses these values to calculate a numericsum for each sample. These numerical values of each sample are in turnsummed to provide a summation or integration of all of the samples underthe pulse width modulated waveform. The summation value of FIG. 22 isninety. This number would be much larger in actual practice. The digitalsignal processor then uses its prestored program to look up the range ofsummation values stored in its lookup tables as described below indetail in conjunction with FIG. 25. Because of signal distortions, whichare always present in a wireless environment, the lookup tables containfinite boundaries or numeric ranges that pertain to each of the sixteenpossible binary combinations. FIG. 22 illustrates that for a value ofninety the four bit combination of zero, one, zero, one is obtained. Anysummation within the numeric range of eighty-five to ninety-five isrepresented in subsequent signal processing of the serial information bythe aforementioned four bit combination.

Like the example discussed above involving multiple phase modulation,products using digital modulation will have prestored ranges dependingupon the design of the radio frequency receiver. If very low receivedvoltages are summed, smaller summation ranges are obtained.

FIGS. 23A and 23B illustrate the sample processing of a half of a cycleof a pulse width modulated squarewave to eliminate the effects of noisewhich introduces error into the calculation of the integral of the halfa cycle as described above in conjunction with FIG. 22. FIG. 23A showsthe leading edge of the waveform that contains a noise transient. Thisnegative going transient is not a portion of the actual pulse widthmodulated data and introduces error in the integration of the waveformby the digital signal processor. Sample signal processing is utilized toassist in the reconstruction of the pulse width modulated waveform toremove transients that are caused by noise and other manmadeinterference. While the digital signal processor is decoding the pulsewidth modulated waveform to transform the serial information into aseries of numerical values each representing the range containing thecalculated integral of each selected part, the numeric sample valuesencoded as groups of bits are stored in a temporary RAM memory. Asillustrated in FIG. 23A, each of the samples is converted to a numericalvalue by an A to D converter 422 associated with the digital signalprocessor. The ROM associated with the digital signal processor stores atable of numerical ranges which represent valid sample values over theduration of a part of the cycle of the subcarrier which are to beincluded in the integration of the subcarrier. As illustrated, thenumerical ranges are based upon expected ranges which occur for aparticular radio frequency receiver design that represent signal levelswhich occur when the half of the subcarrier cycle is at its high or lowlevel. For example, the illustrated transient is outside the numericalrange of sample values which represent valid samples when the pulsewidth modulated carrier is at its high level. When a sudden or dramaticchange in the A to D voltage reading occurs, as described above by thecomparison of the sample value with a range of valid sample values, thedigital signal processor is triggered to perform a series ofcalculations. Because of storage in a RAM buffer area of the samplevalues necessary to compute the integral, one or more sample valuesimmediately before and immediately after a transient are used for signalprocessing to provide a replacement sample value. The replacementinformation is a function of sample values adjacent the sample valuewhich is replaced. In one form of possible signal processing to replacethe noise with a sample value more accurately representing what theactual sample values should have been, the immediately preceding andsucceeding sample values are added and divided by the number of samplesto be averaged to yield fill placement sample value average to fill inthe erroneous sample caused by the noise transient. The resultingwaveform appears in FIG. 23B as a small step that makes the resultingwaveform more representative of the pulse width modulated waveform. Inthis example, if the preceding sample value from the A to D converterwas 1 volt and the following reading was 1.1 volts, the replacementsample would have a value of 1.05 volts. This is considerably moreaccurate than the actual received pulse width modulated waveform thatwould have had a near zero value for the sampling period.

FIGS. 24A and B illustrate the reconstruction of a data waveform whenmodulation of the sinusoidal subcarrier is used as illustrated in FIGS.10A and 11. In this example, the 45° phase being processed is modulatedwith binary information having noise riding on the data signal level. Asdiscussed above in conjunction with the processing of a pulse widthmodulated waveform having noise riding on the data signal level, thedigital signal processor stores the sample values in the temporary RAMbuffer. As illustrated in FIG. 24B, each of the samples is converted toa numerical value by the A to D converter 422 associated with thedigital signal processor. The ROM associated with the digital signalprocessor stores a table of numerical ranges which each represent validsample values over the duration of a part of the cycle of the subcarrierwhich are to be included in the integration of the subcarrier. Asillustrated, the numerical ranges are based upon expected ranges whichoccur for a particular radio frequency receiver design that representsignal levels which occur around the modulated phases of the subcarrier.For example, the illustrated transients are outside the numerical rangesof sample values which represent valid samples when the subcarrier ismodulated with a one or zero as illustrated in FIG. 11 in the 20° windowcentered at 45°. When a series of voltage readings do not conform to therate of rise or slope that would have been typical of valid binaryencoding phase data, the signal processing is triggered to attempt tocorrect the data. The previous and subsequent voltage readings of the Ato D converter 422 are added together and divided by the number ofreadings to substitute a more accurate sample value which wouldtypically be present in the absence of noise for the sample valuerepresenting noise. As can be seen in FIG. 24B, the modified signalwaveform resembles more closely and more accurately the actualtransmitted data. When the digital signal processor now begins theintegration process to determine if the phase information contained atthe 45° phase sample is a binary one or zero, the accuracy of theintegration (and, therefore, the determination) is considerably moreaccurate. FIG. 21A illustrates what the data would look like whensubcarrier modulation is being transmitted. In FIG. 21A it can be seenthat the binary value of the data at the 45° phase is a binary zero andthe binary value of the data at the 135° phase is a binary one. When theradio frequency receiver 12 is located in an extremely noisy environmentthe aforementioned sample signal processing will serve to enhance andreconstruct the received data and will reduce the amount of errorintroduced by noise in the integrating process.

FIG. 25 illustrates the processing of the digital signal processor whichnumerically compares each of the calculated integrals with a pluralityof stored ranges which ranges each represent one of a plurality ofpossible numerical values that the selected part (one-half of asquarewave subcarrier or angular position of an analog subcarrier) mayencode to identify a stored range numerically including the calculatedintegral and substituting for the at least one selected part of each ofthe cycles the one of the plurality of numerical values representativeof the identified stored range including the calculated integral witheach numerical value encoding at least a part of a data unit of theframes of information after the integrated value of the at least oneselected part of a cycle of a subcarrier for a plurality of cycles hasbeen determined which includes the integration of FIGS. 21A and B and 22and the noise transient reduction of FIGS. 23A and B and 24A and B. Thedigital signal processor takes the obtained integrated value and looksup the resulting binary value of a single bit or a group of bitsdepending if the subcarrier modulation is analog or digital orequivalent in the prestored lookup tables. With reference to FIG. 25,the processing proceeds from step 151 where integration is completed todecision point 153 where a determination if the modulation is analog(multiple phase at spaced apart angular positions of the subcarrier ofFIG. 10A) or digital (pulse width modulation of halves of the squarewavesubcarrier of FIG. 10B) is made. If the answer is "yes" at decisionpoint 153, processing proceeds to step 155 where the lookup tables forprocessing the integration of pulse width modulation of a half of acycle of the subcarrier are accessed. The stored ranges are each 100 inmagnitude. Processing proceeds to step 157 where a determination is madeif the value of the integration is less than 900. A value at decisionpoint 157 of less than 900 indicates that the pulse width modulatedwaveform has an inherent problem making the comparison process invalid.If the answer is "yes" at decision point 157, the processing proceeds tostep 159 where an error code is stored in a buffer within the RAM.Processing proceeds from step 159 to decision point 161 where adetermination is made if all of the stored integration values which arebeing group processed have been processed. If there are more values tobe processed, the program loops back to step 155. Otherwise, theprocessing is complete. If the answer at decision point 157 is that theintegral value is not less than 900, processing proceeds to decisionpoint 163 where a determination is made if the integral is less than1100. If the answer is "yes" at decision point 163, a four bit binaryvalue of 0000 is stored at step 165 in the buffer RAM which representsat least a part of an information unit of the serial information.Processing proceeds to decision point 167 where a determinationanalogous to decision point 161 is carried out. If the answer is "no" atdecision point 163, processing proceeds to decision point 169 where adecision is made if the integral value is less than 1200. If the answeris "yes" at decision point 169, processing proceeds to step 171 where abinary value of four bits of 0001 is stored in the buffer RAM. Theprocessing proceeds to step 173 which is analogous to decision point167. The broken line labelled "ONE TEST FOR EACH BINARY VALUE" indicatestesting of the integral values for a series of increasing ranges whichare increased in steps of 100 to determine if the binary valuesrepresenting four bit groups between 0010 and 1110 should be stored inthe buffer RAM. Decision point 175 represents the last test where adetermination is made if the integration value is less than 2600. If theanswer is "yes", the processing proceeds to step 177 where the four bitbinary valve 1111 is stored in the buffer RAM. The processing proceedsfrom step 177 to decision point 179 which is analogous to decisionpoints 167 and 173. If the answer is "no" at decision point 175,processing proceeds to step 181 where an error code is stored in thebuffer RAM indicating that the integration value is greater than thatwhich would be predicted by the prestored values (ranges) for each ofthe sixteen binary combinations. The processing then proceeds todecision point 183 which is analogous to decision points 167, 173 and179.

If the answer at decision point 153 is "no", the processing proceeds tostep 185 where the range for the binary values of one and zero areaccessed for comparison with the integration value obtained at step 151for the modulated separated angular phases of the subcarrier. The binarylookup tables are different than the pulse width modulation tables andare representative of the boundary between "1" and "0" values present inFIG. 21A for each of the separated angular phases which are modulated onthe subcarrier. The integrated value falls within a range on one or theother side of the boundary for each separated angular phase whichcontrols whether the modulation of the subcarrier at the separatedangular positions is decoded as a one or a zero. When the integrationprocess is completed, the processing compares the integrated value withranges that define on which side of the boundary the actual integrationlies. In this process the processing proceeds to decision point 187where a determination is made if the value of the integral is less than350. If the answer is "yes" the processing proceeds to step 189 where abinary zero is stored for the angular phase in a buffer RAM. Theprocessing proceeds to step 191 where a determination is made if morevalues are to be processed. This step is analogous to steps 161, 167,173, 179 and 183 previously described. If the answer is "no" at step187, processing proceeds to decision point 193 where a determination ismade if the value of the integral is less than 700. If the answer is"yes", processing proceeds to step 195 where a binary one is stored in abuffer RAM. The processing proceeds from step 195 to decision point 197where a decision is made analogous to decisions 161, 167, 173, 179, 183and 191 described above. If the answer is "no" at step 193, theprocessing proceeds to step 199 where an error code is stored in thebuffer memory analogous to steps 159 and 181 as previously described.The processing proceeds from step 199 to decision point 201 which isanalogous to decision points 161, 167, 175, 179, 183, 191 and 197.

The contents of the buffer RAM store a group of binary valuesrepresentative of individual bits when multiple phase modulation atseparated angular positions is modulated on the subcarrier and groups ofbits representative of the possible modulated numerical values whenpulse width modulation is modulated on the subcarrier. The contents ofthe buffer RAM store the detected serial information containing thedetected IDENTIFICATION FRAME GROUP or modifications thereof forsubsequent processing by the digital signal processor. Any errors causedby fading or other transmission faults which render one or more bits ofindividual frames erroneous and uncorrectable or a sequence of framesincluding whole frame groups which are erroneous are contained at thistime in the buffer RAM. The digital signal processor detects when anerror is present in each frame by processing the error correction codeembedded in the frames of the stored serial information as describedbelow.

Although the previously described sample processing will serve to removetransients that may produce the decoding of erroneous data when largeerrors are introduced into the calculation of the integrals, it is stillpossible that the integration of the data modulated on the subcarrier ata particular phase would result in an erroneous detection. Manydiscriminators in radio receiving electronics have finite voltage limitswhen data is being detected. When the radio frequency receiver isdesigned for low voltage operation, the recovered data will be betweenzero and one volt in amplitude. However, in many types of discriminationthere are particular combinations of interferences (typically, adjacentchannel interference) that can cause a noise signal to be much greaterin amplitude than the one volt level. These spikes or noise may be ashigh as two or three times the expected amplitude and not berepresentative of a true received data signal. The problem is moreprevalent when multiple phase data is being decoded as this type ofadjacent channel noise that is detected by the discriminator contributesgreatly to distorting of the detected waveform and may change a binaryzero to a binary one and a binary one to a value much greater than whata binary one is predicted to be. As previously described, the samplesignal processing has finite limits on an amount of data interpretationthat can be accomplished. Specific high and low boundaries must beplaced in the lookup tables to prevent such data interpretation frombeing considered invalid. This is the reason for finite boundary valuesas discussed above in processing both multiphase and pulse widthmodulation of the subcarrier. The boundaries and the need for suchboundaries will be dependent upon the receiving circuitry design of theparticular product. Therefore, the boundaries represented by decisionpoints 159, 181 and 199 may or may not be necessary in the receivingcircuitry of a particular multiple phase or pulse width modulationapplication of the receiving circuitry which can make steps 159, 181 and199 unnecessary. If the receiving circuitry is based exclusively uponeither the multiphase or pulse width modulation protocol of FIGS. 10Aand B, decision point 153 may be omitted with only the necessary part ofthe processing for the particular protocol being included in thereceiving circuitry.

FIG. 26 illustrates a representation of bits of the fourth and fifthframes of the IDENTIFICATION FRAME GROUP in accordance with FIG. 9,after detection of the transmitted radio frequency carrier anddemodulation of the subcarrier including the processing of FIG. 25. Thebits of the error correction field are discarded when decoding iscompleted without any erroneous uncorrectable bits. This leaves thedecoded bits for subsequent processing such as outputting of the dataunits or data bits for determining if the decoded identification codematches one of the identification codes of the radio frequencytransmitters 14, 16 and 18 which the radio frequency receiver 12 isprogrammed to track and further information such as, but not limited to,the status of the "panic" switch 114. The data bits of FIG. 26 are allvalid data bits which do not require reconstruction by the radiofrequency receiver as described below in conjunction with FIGS. 27-29.As is illustrated in FIG. 26, a broken vertical line in the left-handportion of FIG. 26 indicates a break in the time base between bits 2 and7 in the tenth data unit. The upper series of numbers in the horizontalrow of boxes, as indicated above, identifies bit positions within thefourth frame. The lower boxes containing the legend "V", which are forillustration purposes only, identify that the data is valid whichsignifies that the frame has been processed with the error correctioncode and no data bits within the frame have been found to be invalidbeyond the bit error correction capacity of the error correction code.It should be understood that the use of the identifying letter "V" isnot actually stored in the memory associated with the digital signalprocessor. The error correction code bits have a value which is afunction of the bits of the data units contained in the frame. Theactual value of the data bits and the functionally related errorcorrection code bits has not been shown because it is not necessary forunderstanding the invention. In summary, FIG. 26 illustrates an exampleof the stored valid data which occurs when the error correction codecapability of a frame is not exceeded, i.e. all bits are valid in theIDENTIFICATION FRAME GROUP of FIG. 9B stored in the radio frequencyreceiver RAM after processing with the error correction code iscompleted.

FIGS. 27-29 illustrate frames which contain at least one erroneousuncorrectable bit. As illustrated in FIGS. 27-29, like in FIG. 26,vertical wavy lines indicate time breaks between bit positions of aparticular frame. The top horizontal row of numbers in FIGS. 27-29, likein FIG. 26, identify particular bit positions within the data units andwithin the error correction code of a frame within an IDENTIFICATIONFRAME GROUP of a format of FIG. 9. The bottom series of letters use a"V" to identify valid data, and an "E" to identify erroneous bits whichcannot be corrected by the processing of the bits of the frame witherror correction code. It should be understood that the use of theidentifying letters "V" and "E" are only for illustrative purposes andare not actually representative of data stored in the memory associatedwith the digital signal processor which, of course, is bit values of oneor zero. Again, like in FIG. 26, knowledge of the actual value of thedata units and error correction code is not necessary to understand theexamples of FIGS. 27-29 illustrating erroneous uncorrectable bitpatterns comprised of bits identified by the letter "E". Typically, theBCH 45,24 error correction code which is used with the protocol of FIG.9 has the ability to correct up to two bit errors per frame. With theprior art, the presence of erroneous uncorrectable bits results inerroneous information because there was no processing capabilityprovided in the receiving circuitry receiving a wireless transmission ofinformation to recover erroneous bits after the error correctioncapacity of the error correction code is exceeded as is indicatedsymbolically by the letter "E" in FIGS. 27-29.

The error recovery and reconstruction capability of the presentinvention is based upon the processing capability of at least oneprocessor within the embodiment 400 radio frequency receiver 12, whichpreferably is at least one digital signal processor as illustrated inFIG. 15, to detect erroneous bit patterns in the field of the errorcorrection code bits after processing of the frame with the errorcorrection code. The erroneous bit patterns either contain a series ofall zeros or all ones of a number exceeding the bit error correctioncapacity of the error correction code. That is, if the BCH error codebit error correction capacity is two bits, a pattern of at least threeor more all zeros or all ones would be the object of the pattern search.Once the error correction code has been processed in each frame and thecomputation result indicates that at least one erroneous bit is present,which signifies exceeding of the error correction capability of theerror correction code contained in the frame, the digital processorsearches the stored bits to look for the aforementioned erroneous bitpattern of all zeros or all ones located totally within the errorcorrection bit field. Detection of these patterns and their positionwithin the stored bits in memory by bit shifting or other knowntechniques after computation by the digital signal processor that atleast one erroneous uncorrectable bit is present in a frame is used todetermine in which bit positions the erroneous uncorrectable bits arepresent. If these bit patterns are found to be totally within the errorcorrection code bit field, valid bits outside the bit field of the errorcorrection code (data) are recovered and reconstructed as explainedbelow in conjunction with FIG. 27 with the error correction code bitsbeing discarded. If the pattern of all zeros or all ones is not found tobe totally within the error correction code bit field, the data bitscannot be recovered and reconstructed which requires that furtherprocessing of the data bits of the frame not be undertaken. Anuncorrectable error in the identification code will disqualify the useof the RSSI signal produced by that transmission. However, if anerroneous uncorrectable bit is present in frame four, which contains thebit field CB used to encode commands and the status of the "panic"switch 114, the resultant RSSI signal will be further processed todetermine if it should be used as part of the average computationprocess as described above and below. All of the frames of theIDENTIFICATION FRAME GROUP of FIG. 9 may be reconstructed to recoverotherwise erroneous uncorrectable data bits. Recovery of data bits,which would be erroneous when error correction code is the exclusiverecovery mechanism, facilitates the ranging and tracking process byqualifying the greatest number of RSSI signals for subsequent processingas described above qualifying the greatest number of receptions of theidentification code.

The digital signal processor processes the stored bits of the dataframes within the IDENTIFICATION FRAME GROUP with the error correctioncode therein to determine if the plurality of bits of the frames do notcontain any erroneous uncorrectable bits which dictates that the data bestored as valid data and the error correction code be discarded. If atleast one erroneous uncorrectable bit signified symbolically by theletter "E" in FIGS. 27-29 which cannot be corrected with the errorcorrection code is located, the digital signal processor processes thestored bits of the frames which contain the at least one erroneousuncorrectable bit somewhere therein to determine if the frames containonly valid data bits in the data field signified by the erroneous bits(the aforementioned multibit pattern of zeros or ones) being totally inthe error correction code field which is illustrated in FIG. 27 whichrenders the data bits valid and the error correction is discarded.

As is illustrated in FIGS. 28-29, all of the data bits are not valid assymbolically identified by the letter "E" outside the error correctioncode bit field which renders the data bits of the frames of FIGS. 28 and29 invalid. In FIGS. 28-29, the pattern of erroneous uncorrectable databits identified by the letter "E" is not totally contained in the errorcorrection code bit field which makes it impossible for the digitalsignal processor to discriminate whether or not any of the data unitscontain valid data. It is not possible to determine reliably whether anyof the eight bit data unit bit groups illustrated in the IDENTIFICATIONFRAME GROUPS of FIGS. 28 and 29 are valid data when erroneousuncorrectable bits are not totally present within the error correctioncode, as, for example, being totally contained in the data units in FIG.28 or spanning the error correction bit field and the data unit bitfield as illustrated in FIG. 29.

The process of determining whether valid data can be reconstructed fromframes of the IDENTIFICATION FRAME GROUP containing at least oneerroneous uncorrectable bit by processing the error correction code ofthe frames can only be successfully performed in situations when minorfades or transmission errors occur where synchronism is not lost andwhen the bit error correction capacity of the error correction code isexceeded. As illustrated in FIG. 27, only the circumstance when theerror correction code bit field is determined by the aforementionedpattern recognition capability of the digital signal processor tototally contain a successive pattern of all zeros or all ones, such atleast three successive bits when the BCH code is capable of correctingfor a two bit error, represents recoverable and reconstructible data.

After the reconstruction is complete, there no longer is a need forprocessing the error correction code bits. Thereafter, the errorcorrection code bits are discarded and only the bits of the data unitsof the frames (bits other than error correction code) are stored inmemory for further processing to identify if the radio frequency carriercontained a valid identification code of a radio frequency transmitterand what the status of the "panic" switch 114 is and any otherinformation from the radio frequency transmitters 14, 16 and 18 whichare assigned to the radio frequency receiver for tracking or monitoringfunctions, etc. Thereafter, processing of the RSSI signals and thestatus of the "panic" switch 114 as described below is preformed by thedigital signal processor.

The radio frequency receiver embodiment 400 must perform a multiplicityof functions in order to reliably monitor and track the transmitters 14,16 and 18. Battery longevity is an important concern. The radiofrequency receiver embodiment 400 and the radio frequency transmitterembodiment 100 are designed to be a portable product with the batterylifespan being maximized by the operating software of the digital signalprocessors contained in the radio frequency receiver and radio frequencytransmitter by performing numerous power management functions. The powermanagement functions of the radio frequency transmitter embodiment 100have been described above. In the radio frequency receiver embodiment400 only those circuits which need to be in operation at a given timeare turned on to conserve battery lifespan.

FIG. 30 is a flowchart of the operation of the radio frequency receiverembodiment 400 including battery conservation and initializationtechniques. Operation proceeds from the turning on of the power at point501 to point 503 where the control CPU 402 is reset. At point 505 thepotential of the batteries is read. Processing proceeds to decisionpoint 507 where a determination is made if the battery voltage as readat point 505 is sufficient to provide sufficient power to begin thereceiving process. If the answer is "yes" at decision point 507,processing proceeds to point 509 where the digital signal processorcauses the alert 428 to emit warning beeps. If the answer is "no" atdecision point 507 or the warning beeps have been emitted at point 509,processing proceeds to point 511 where the digital signal processorcauses a check to be made of the factory programmed inputs for theoperational parameters of the radio frequency receiver embodiment 400.These operating parameters include the specified series of frequencies(e.g. fifty) on which the radio frequency receiver embodiment 400 willreceive the IDENTIFICATION FRAME GROUP of FIG. 9 from each of the radiofrequency receivers 14, 16 and 18 that are being monitored by the radiofrequency receiver. The digital signal processor commences at point 513to program the first radio frequency carrier frequency by sending aserial stream of digital data to the synthesizer and phase lock loop404. Upon programming the start frequency, the digital signal processorturns on the voltage controlled oscillator 410 as indicated at point515. The operation proceeds to point 517 which is wait period duringwhich the digital signal processor looks to receive the lock on signalfrom the phase lock loop of the synthesizer/phase lock loop 404.

The lock on time of the phase lock loop of the synthesizer/phase lockloop 404 may vary depending upon the components of the loop filter aswell as the battery voltage. As the battery voltage drops, the lock ontime becomes progressively longer until, at some point in time, afrequency lock on condition cannot be achieved. This is due to the factthat the batteries no longer have sufficient voltage to provide thenecessary power to the voltage controlled oscillator 410 (and othercircuits) to maintain the radio frequency receiver embodiment 400 in anoperational status. Processing proceeds to decision point 519 where adetermination is made if the lock on signal has been received. If theanswer is "yes" at decision point 519, processing proceeds to point 521where the intermediate frequency amplifier in the bandpassfilter/intermediate frequency amplifier 416 is turned on. Processingproceeds to point 523 where a set delay of a number of milliseconds isallowed to expire to provide sufficient time for the intermediatefrequency amplifier of the bandpass filter/intermediate frequencyamplifier 416 to come up to an operational status. Processing proceedsto decision point 525 where a determination is made if a RSSI signal isbeing outputted by the bandpass filter/intermediate frequency amplifier416. If the answer is "yes" at decision point 525, processing proceedsto point 527 where the IDENTIFICATION FRAME GROUP is decoded includingdemodulating the identification code of the transmitter and the statusof the "panic" switch 114 of the radio frequency transmitter embodiment100 as encoded in the field CB of the IDENTIFICATION FRAME GROUP.Processing proceeds to decision point 529 where a determination is madeif the battery voltage is low. If the answer is "yes" at decision point529, processing proceeds to point 531 where warning beeps are caused tobe emitted by the alert 428. If the answer is "no" at decision point 529or warning beeps have been emitted at point 531, processing proceeds topoint 533 where the digital signal processor 402 begins an orderly shutdown process of unnecessary receiving circuits which consume power andbegins the analysis of the data contained in the CB field, as well asthe processing of data units 1-5, if the data units 1-5 of theIDENTIFICATION FRAME GROUP contain any necessary data for the operationof the radio frequency receiver embodiment 400. The embodiment 400 doesnot use data units 1-5 to perform range monitoring and directionaltracking. If the answer was "no" at decision point 525 that no RSSIsignal is being outputted by the bandpass filter/intermediate frequencyamplifier 416, processing proceeds to the power shutdown point 533 asdescribed above. If no RSSI signal voltage is outputted by the bandpassfilter/intermediate frequency amplifier 416, the digital signalprocessor immediately begins the power down sequence. The presence ofthe RSSI voltage is an indication that a transmitted radio frequencycarrier is present and therefore, the decoding process should beenabled. If the RSSI voltage is not present, this is an indication thatthere is no longer a need for the radio frequency receiver embodiment400 to remain on as none of the radio frequency transmitters 14, 16 and18 are transmitting at this time.

If upon successfully receiving the identification code and the statuscode, contained in the field CB of the IDENTIFICATION FRAME GROUP, thedigital signal processor examines the field CB to see if an alert statushas been received which is caused by the user of the radio frequencytransmitter closing the "panic" switch 114. Processing proceeds frompoint 533 to decision point 535 where a determination is made if thefield CB of the IDENTIFICATION FRAME GROUP contains an indication of a"panic" status produced by the user of the radio frequency transmitterclosing the "panic" switch 114. If the answer is "no" at decision point535, processing proceeds to 537 where the digital signal processorselects the next frequency of the staircase sequence of radio frequencycarrier frequencies on which the radio frequency receiver embodiment 400is receiving transmissions. If the answer is "yes" at decision point535, processing proceeds to point 539 where a change in the status codeis made and the digital signal processor produces alert beeps with thealert 428. Processing proceeds to point 541 where a waiting period isentered permitting the user of the radio frequency receiver to close the"find me" switch 426. Closing of the "find me" switch 426 by the user ofthe radio frequency receiver embodiment 400 causes the digital signalprocessor to change its software routine to convert the antenna array405 to a directional array, as described above, and to further activatethe LCD or LED display 424 to display the magnitude of each successiveRSSI signal, which is preferably the integral thereof, as part of thetracking process as described above in conjunction with FIG. 2. Theprocessing proceeds from point 541 to point 537 where the next receivedfrequency is programmed into the synthesizer/phase lock loop 404. If theanswer is "no" at decision point 519, processing proceeds to decisionpoint 543 where a determination is made if the battery voltage is low.If the answer is "yes" at decision point 543, processing proceeds topoint 545 where the digital signal processor causes warning beeps to beemitted analogous to those admitted at point 531. If the answer is "no"at decision point 543 or warning beeps have been emitted at point 545,processing proceeds to decision point 547 where a determination is madeof whether a time interval has elapsed which signifies that the radiofrequency receiver embodiment 400 cannot lock onto the commandedfrequency. If the answer is "no" at decision point 547, processingproceeds back to decision point 519 as described. If the answer is "yes"at decision point 547, processing proceeds to point 549 where warningbeeps are emitted which are analogous to the warning beeps at points 545and 531 as described above.

The lowest operating voltage of the batteries is obtained when all ofthe electronics are turned on including the LCD or LED display 424 (ifthe turning on of all of the electronics causes the voltage to dropbelow the minimum threshold, the digital signal processor begins thebattery low alerts to indicate to the user that the batteries are inneed of recharging).

When the receipt of a valid identification code has been verified, alarge number of samples are taken of the RSSI signal voltage produced bythe output of the intermediate frequency amplifier which is part of thebandpass filter/intermediate frequency amplifier 416. For example, ifthe transmitted duration of the IDENTIFICATION FRAME GROUP is 100milliseconds, thirty to forty RSSI samples may be taken during thisperiod. This integration process tends to cancel out the rapidlyfluctuating electrical noise which rides on the top of the average valueof the RSSI signal. The electrical noise is a product of the environmentin which the radio frequency receiver 400 embodiment is operating.

FIG. 31 illustrates a typical voltage fluctuation in a RSSI signalproduced during the reception of the IDENTIFICATION FRAME GROUP which ispreferably integrated to remove the rapidly varying noise which isindicated by the solid rapidly varying line illustrated in FIG. 31. TheRF environment of the radio frequency receiver embodiment 400 istypically hostile and, as illustrated, the average RSSI signal amplitudealso varies more slowly in amplitude due the effects of Rayleigh fadingand multipath signals as indicated by the dotted line. The more slowlyvarying noise would contribute significantly to erroneous calculation ofthe transmitter's range if the effects of this noise were not eliminatedby averaging or integration of the RSSI signal over the entire samplingperiod T, as described above, where a running average of successiveintegrated RSSI signal samples for each of the radio frequencytransmitters 14, 16 and 18 is made by the digital signal processor inorder to determine if any one of the radio frequency transmittersexceeds the set range 20 as described above. The integration oraveraging process which yields the true integral value over the sampleinterval T, by taking numerous samples, removes the rapidly and slowlyvarying electrical noise to produce an integrated value as indicated bya solid horizontal line of the RSSI signal which does not contain theeffects produced by Rayleigh fading, etc. The solid line represents theactual integrated value of all of the samples over the entire samplingperiod T which corresponds to the time of reception of theIDENTIFICATION FRAME GROUP. Each of the multiple samples are taken inrelation to each other to provide the actual voltage variationrepresented by the dotted line in FIG. 31. Upon completion of thesampling period T, all of the samples are summed and divided by thenumber of samples to provide the average or integration value over thereception period of the IDENTIFICATION FRAME GROUP period as indicatedby the solid horizontal line.

The rationale behind the averaging process performed by the integrationas described above in conjunction with FIG. 31 is that the radiofrequency transmitter is designed to be worn by a small child andtherefore, only relatively small changes in the average RSSI signal willoccur as a consequence of actual motion of the child. As has beenexplained above, the broadcast of the identification code may occur at afrequency of up to ten times per second which means that the relativemotion which could occur between the successive transmissions by theradio frequency transmitter is small.

As is apparent from FIG. 31, over the sample period T, significantvariation occurs in the RSSI signal which is not caused by motion of theuser of the radio frequency transmitter embodiment 400. In fact, quitetypically, the entire reading may be averaged to be higher or lower thanthat which would be representative of the actual distance of the radiofrequency transmitter 14, 16 or 18 from the radio frequency receiver 12due to the effects of Rayleigh fading and multipath interference. Asecond integration or averaging of the individual RSSI integrations,each represented by the horizontal solid line in FIG. 31, is necessaryto most reliably determine the distance of the radio frequencytransmitters 14, 16 and 18 from the radio frequency receiver 12.

During the receiving process, the digital signal processor is performingtwo simultaneous tasks. The first is the analysis of the RSSI signalsand the second is the verification of the identification code containedin the IDENTIFICATION FRAME GROUP. The digital signal processor mustbegin the RSSI signal sampling process immediately upon the onset of thesignal reception. However, during this period of time, the radiofrequency receiver 12 is unaware if the RSSI signal belongs to atransmitter which is being monitored by the radio frequency receiver. Itis not until synchronization between the at least one transmitter 14, 16and 18 and the receiver 12 is achieved under control of the digitalsignal processor and the receipt of a statistically reliable number ofthe digits of the identification code as described below or the entireidentification code which is produced by the decoding of theidentification code contained in the IDENTIFICATION FRAME GROUP that adetermination can be made by the digital signal processor that the RSSIsignal indeed corresponds to that of one of the monitored radiofrequency transmitters 14, 16 and 18. If a match of the identificationcode does not occur, the averaged (integrated) RSSI signal data takenduring the sampling of the RSSI signal is discarded. Only whenverification occurs that the RSSI average (integration) data indeedbelongs to one of the radio frequency transmitters 14, 16 or 18 beingmonitored, is the RSSI average data stored in a RAM memory of the radiofrequency receiver embodiment 400.

In order to obtain the most reliable distance information from the RSSIsignal, a second integration or averaging process is performed whichremoves the effects of time variation on each integrated RSSI signalover the sampling interval T of FIG. 31 not representing the truereceived signal strength because of the effects of fading, etc. Thedotted line in FIG. 32 represents the value of the integration of theRSSI signal of FIG. 31 which would occur at any instant in time as afunction of distance. Over time for a fixed distance, the value of theindividual integrations of each radio frequency carrier transmissioncontaining a valid identification code as illustrated in FIG. 31 wouldvary on both sides of the solid line. Thus, the dotted line will varyover time and, at any single point in time, represents at any fixeddistance the instantaneous value of each integration of FIG. 31. Thesecond integration or averaging represented in FIG. 32 removes theeffects of this time variation on the magnitude of the integrations ofFIG. 31 so that the time averaged or integration of the integratedsamples of FIG. 31 represented by the solid line is purely a function ofdistance.

Furthermore, as explained above, each successive integrated RSSI sampleis first tested to make sure that its reading is not above or below acertain predetermined function, as described above, which is indicativeof a Rayleigh fade or multipath interference or other signal degradingphenomena. If the most current integrated RSSI signal is above or belowthe previous integrated average by a function, such as twenty percent,of the average of the integrated RSSI signals, the sample is discardedand a number of previous samples, such as five samples, are utilized tocompute the average. This has the net effect of removing the fading andmultipath components that are present in each RSSI sample as indicatedby the time fluctuating dotted line in FIG. 32.

The RSSI signal voltage is representative of the amount of radio voltagepresent at the input of the radio frequency receiver embodiment 400 asapplied to the low noise amplifier 407. The RSSI signal voltage isessentially linear and is a very accurate indication of the distancebetween the radio frequency receiver 12 and the radio frequencytransmitters 14, 16 and 18. The aforementioned double processing stepsof integrating or averaging the individual samples and then furtherintegrating or averaging the samples to produce an average which iscompared to the output voltage produced by the range control 420 permitsan extremely accurate monitoring of distance to be made which permitsthe user of the radio frequency receiver 12 to accurately determine ifany of the radio frequency transmitters 14, 16, or 18 have moved outsidethe set range 20. The radio frequency receiver 12 has the ability toperform this range determination due to the fact that the output powerfrom each of the radio frequency transmitters 14, 16 and 18 is known andconstant as a consequence of their design. Therefore, the RSSI signals,as processed as described above to remove the effect of noise, aredirectly representative of range information.

This mode of operation is different than ranging systems where the powerof the transmitter is typically not known and, therefore, littlecredibility can be given to a RSSI signal as the basis for measurementof a distance between a radio frequency receiver and a radio frequencytransmitter.

As is illustrated in FIG. 32, the particular embodiment as describedwill have a RSSI voltage which varies between approximately 0.5 and 2.5volts. This corresponds to a working range between a few feet out to andexceeding 1000 feet of separation between the radio frequency receiver12 and the radio frequency transmitters 14, 16 and 18 as discussedbelow. The double integrated or averaged RSSI voltages are used forcomparison by the digital signal processor to provide the range anddirection control of the radio frequency receiver 12.

The user of the radio frequency receiver 12 uses the variable rangecontrol 420 to set the variable distance 20 which determines when theradio frequency receiver 12 generates an alert for the benefit of theuser that one or more of the transmitters 14, 16 or 18 have movedoutside the set range. As has been explained above, the range control420 produces a variable range voltage that is presented to the digitalsignal processor for comparison with the averaged RSSI signals asdescribed above. The digital signal processor is constantly comparingthe present voltage representing the set range 20 produced by the rangecontrol 420 to the average RSSI voltage which, preferably, is processedwith the double integrations, or averages, as described above. Forranges less than approximately fifty feet, the RSSI voltage may becomesomewhat non-linear, but for ranges exceeding fifty feet, the RSSIvoltage will be substantially linear.

When the alert 428 of the radio frequency receiver 400 generates analert, the user of the radio frequency receiver is alerted that one ofthe radio frequency transmitters has exceeded the set range 20. Thefollowing steps are taken which have been described above generally withrespect to FIG. 2. The user of the radio frequency receiver 12 typicallywould remove the unit from a belt and hold the unit in such a positionthat the LCD or LED display 424 is readily visible and depress the "findme" switch 426. The digital signal processor senses that the "find me"switch 426 has been closed and changes its operating mode to provide adynamic display of each successive RSSI signal which as described aboveis preferably integrated to remove the effects of noise. There is noneed at this point for the second integration or averaging process, asdescribed above in conjunction with FIG. 32, because it is onlynecessary to have constantly updated integrated RSSI samples inaccordance with the solid line of FIG. 31 which are indicative of anytrue relative motion between the radio frequency transmitter 14, 16 or18 being monitored and the radio frequency receiver. As explained above,the digital signal processor changes the antenna array 405 from anomnidirectional to a directional antenna which permits the radiofrequency receiver to orient its received beam width which isrepresented by the axis 26 of FIG. 2 to a very narrow angle. The user ofthe radio frequency receiver then can physically rotate the receiver asillustrated in FIG. 2 to an orientation 22 which maximizes the displayproduced by the LCD or LED display 424. The user of the radio frequencyreceiver 12 then walks in the indicated direction from which the maximumsignal strength is being received to find the radio frequencytransmitter 14, 16 or 18 being tracked.

The system 10 is based upon one-way data transmission. A mobile radiofrequency transmitter 14, 16, or 18 located, for example, on a childtransmits its identification code to the radio frequency receiver 12. Inorder to synchronize the radio frequency receiver to the radio frequencytransmitters, the following procedure takes place upon turn on. Thetransmitter is first turned on followed by the radio frequency receiver12. Upon turn on of one radio frequency transmitter, it immediatelybegins its frequency hopping "chirping" and continues to do so at afixed rate. When the radio frequency receiver 12 is turned on, itinitially camps on a single frequency and awaits to receive a chirp codefrom the radio frequency transmitter. When the transmitter code isreceived, the radio frequency receiver 12 then establishessynchronization with the radio frequency transmitter.

The radio frequency receiver 12 will then automatically follow the radiofrequency transmitter by arriving at the next sequenced frequency aheadof the radio frequency transmitter and awaiting to receive theidentification code. Upon receipt of the identification code from theradio frequency transmitter, the radio frequency receiver continues thestepping process to track the radio frequency transmitter through theentire range of spread spectrum frequencies.

When multiple radio frequency transmitters 14, 16 and 18, as illustratedin FIG. 1 are utilized, a similar camp and wait function is performed bythe radio frequency receiver 12 with one slight variation in operationalperformance. The radio frequency receiver 12 measures the time betweenthe two received transmitted signals and then performs a dual ormultiple mode hopping, where it follows each of the sequences of theradio frequency transmitter 14, 16 and 18 correspondingly. Since thereis a finite period of time between transmissions and multiple radiofrequency transmitters typically are slightly offset in their timing,collision avoidance is enhanced and does not become a problem.

Even with a minimal number of chirp codes and a minimal number ofidentification codes (e.g., four each) there is a tremendous resilienceto interference from the radio frequency transmitters 14, 16 and 18.Different chirp codes reduce the probability of interference toapproximately two percent. The identification codes further reduceinterference when the same chirp code is present on numerous radiofrequency transmitters in a given area. This collision interferenceavoidance is further enhanced by the fact that even though multipleradio frequency transmitters 14, 16, or 18 may reside in a given areawith the same chirp code and the same identification code, theprobability of the hop sequences (with fifty frequencies) provides anadditional interference probability of less than two percent. Thisoccurs because the probability of multiple radio frequency transmitters14, 16 or 18 with the same chirp code and identification codes hoppingon the same frequency at precisely the same time is extremely low. Thisinterference resistance is further enhanced by the fact that the captureeffect of the radio frequency receiver 12 will only select the closestradio signal and therefore, minimizes the same frequency interferencesfrom other radio frequency transmitters in a given area.

It has been discovered that after the synchronization of frequencyhopping between the transmitters 14, 16 and 18 and the receiver 12 hasbeen established, the validation of the identification code of each ofthe transmitters under control of the digital signal processor of thereceiver may be based upon a matching of less than all of theidentification code digits of each transmitter to qualify the RSSIsignal for full signal processing as described above which enhances thesensitivity of the receiver providing range and directional tracking.For example, with a four-digit identification code, matching of two orthree identification code digits after synchronization between thefrequency hopping transmitters 14, 16 and 18 and receiver 12 isestablished can be used to qualify statistically reliable RSSI signalswhich provides highly sensitive distance and directional trackinginformation as described above. The number of digits of theidentification code of each transmitter 14, 16 and 18 necessary to bematched less than the full number of identification code digits dependson the application and the number of digits used in the identificationcode. As a result, weak transmissions from the transmitters 14, 16 and18 which are close to the signal to noise limit of the receiver 12 maybe validly processed to enhance the operation of the ranging anddirectional finding functions as described above.

In order to gain insight as to the reliability and rangingcharacteristics of the present invention, an evaluation of thecomponents of the radio signal from the radio frequency transmitters 14,16 and 18 to the radio frequency receiver 12 is made. The final poweramplifier PA2 of the radio frequency transmitter has an output ofapproximately five milliwatts. In the radio environment, typically theseradio powers are expressed in dbm (five milliwatts would equal a +7 dbmpower level).

The antenna in the radio frequency transmitters 14, 16 or 18 is verysmall and is approximately a quarter wavelength. This provides a gain oftypically zero dbm. However, because of the shielding constraints of thehousing of the radio frequency transmitter and the fact that it is wornon a person's belt, the anticipated gain will be -10 dbm. This providesan actual radiated power of -3 dbm at the antenna.

The path loss at 920 MHz. varies proportionally with distance. Althoughthe formulas to support these calculations are not stated herein, theempirical results are illustrated in FIG. 33. FIG. 33 plots the freespace loss in dbm as a function of the distance between the radiofrequency transmitters 14, 16 or 18 and the radio frequency receiver 12.It should be noted that at approximately 100 feet there is 62 dbm pathloss and that increases to approximately 86 dbm at 1700 feet.

The antenna 405 of the radio frequency receiver embodiment 400 has thenet gain of -10 dbm. The radio frequency receiver input sensitivity is a-115 dbm and therefore, when added to the antenna gain (actually aloss), a -105 dbm receiver sensitivity is achieved at the antenna inputterminals.

Mathematically, it can be seen that a -85 db path loss added to a -3 dbloss at the antenna results in a -88 db signal presented to the radiofrequency receiver including antenna loss having the sensitivity of a-115 db. The net result is approximately a 30 db difference over andabove what the radio frequency receiver embodiment 400 needs as anacceptable signal level and therefore, the radio frequency receivershould work to a distance reliably of at least 1700 feet.

Two factors contribute additional loss which are body and buildingattenuation. Attenuation on a human body at 900 MHz. is approximately 10db. The attenuation in a residence (typical, wood, aluminum, or brickstructure) is also 10 db. When collectively added together, anadditional 20 db of loss occurs in the path by the effects of the homeresidence as well as the possibility that a person is facing away fromthe home and therefore, the radio signal must penetrate through the bodyof the user of the radio frequency receiver 12. At a 1700 foot distance,this leaves an adequate signal reserve of 10 dbm.

FIG. 34 illustrates the relationship between the input field strengthand the RSSI signal voltage. This voltage varies between approximately0.5 and 2.5 volts depending upon the received radio field strength. Thiswide dynamic range permits the radio frequency receiver embodiment 400to readily determine the relationship between distance and voltage whenthe double averaging/integration processes described above are used toremove the electrical noise, Rayleigh fading and multipath anomaliesthat typically exist in the RSSI signal prior to processing by thedigital signal processor. There is approximately a 20 db margin whichpermits the radio frequency receiver in a non-noisy environment tooperate at distances approaching a mile.

FIG. 35 illustrates free space loss over a distance of up to one mile.As can be seen, the radio frequency receiver 400 may have the capabilityof operating at a distance as great as one mile providing that there areno additional attenuations to minimize the path loss.

Furthermore, the design of the antennas of the radio frequency receiver12 and the radio frequency transmitters 14, 16 and 18 may be optimizedto emphasize the most accurate range readings for separation distancesbetween 100-900 feet. This monitoring range is adequate for mostdistance monitoring functions involving people such as small children.However, variations may be made to permit tracking up to a distance of amile.

In an extended range version, the radio frequency receiver 12 willimmediately return to a single frequency until the radio frequencytransmitter 14, 16 or 18 identification code is again received toreestablish synchronization. This permits a complete loss of thetransmitted signal to occur and by a person moving around in a searchpattern an attempt may be made to reestablish synchronism with the radiofrequency transmitter 14, 16 or 18 which is being tracked and to beginthe directional tracking process.

FIGS. 36-38 illustrate the present invention being used by a user 600 todirectionally track a transmitter 18 and a preferred design of a housing614 for the receiver unit 12' which enhances the sensitivity of thereception by the receiver unit as a consequence of requiring the user tohold the receiver unit in the user's hand 602 at a position spaced awayfrom the body and in elevated positions preferably at least at chestheight. As illustrated in FIG. 36, the receiver unit 12' may be heldaway from the body in the hand 602 of the user 600 in a range ofelevated positions 611 between waist level and eye level. The range ofelevated positions includes substantially at arm's length at or slightlyabove waist height depending upon the length of the user's arms asindicated in phantom at position 604, away from the body substantiallyat arm's length at or slightly above chest height as indicated in solidlines at position 606 or away from the body as illustrated in phantom atposition 608 substantially at eye level. The transmitter 18 producestransmissions 610 which travel in a line of sight to the receiver unit12' as indicated. The height 611 represents a range of verticalorientations in which the display 424 of FIG. 14 may be viewed. Thesensitivity of reception by the receiver unit 12' is enhanced both bypositioning of it away from the body to provide for reception closer tofree space conditions and further to enhance the height of the receiverunit which also enhances receiver reception sensitivity. As illustrated,it is desirable to space the receiver unit 12' one or more wavelengthsnλ away from the body of the user 600 indicated by the distance 612 inFIGS. 36 and 38 to provide conditions which are more representative offree space reception to enhance the sensitivity of the reception of thetransmissions 610 from the transmitter 18.

Enhancement of the sensitivity of reception of the transmissions 610 ishighly desirable for a low power battery operated unit which operates ata maximum typically of 100 milliwatts or below in accordance with FCCregulations for spread spectrum transmissions. Enhanced sensitivity ofreception of the transmissions 610 provides improved monitoring of thetransmitters 14, 16, and 18 at the greatest possible range from thereceiver unit 12' and further greater capability for performingdirectional tracking of the transmitters 14, 16 and 18 as describedabove.

As illustrated in FIG. 37, the transmitter unit 12' is preferablycontained in a plastic housing 614 which contains the electronicsdescribed above in a surface mounted circuit board including the display424 to provide a visual indication of the magnitude of the receivedsignal strength of the transmissions 610 from the transmitter 18 tofacilitate directional tracking as described above. The detailedelectronics of the receiver 12 of the receiver unit 12' have beenomitted from FIGS. 36 and 37 and preferably are as described above. Asillustrated in FIG. 37, the switch 426 is mounted in the housing 614 andis electrically coupled to the receiver electronics to activate thedirectional antenna function. The switch 426 has a first position atwhich the directional antenna 431 is not operative to receive the radiotransmissions 610 from the at least one radio transmitter 18 and asecond position at which the directional antenna is operative to receivethe radio transmissions from the at least one radio transmitter.

The switch 426 is positioned relative to the housing 614 so that thehand 602 of the user 600, including the thumb 616, holds the switch inthe position to activate the directional antenna 431 such that thedirectional antenna is positioned relative to the housing so that duringthe holding the switch in the second position by the user's hand, a lineof sight of the transmissions 610 exists between the directional antennaand the at least one transmitter which is not occluded by the user'shand as illustrated holding the switch in the second position.

A field of view limiter 618 limits light emanating from the display 424to a field of view of the display when the user 600 holds the receiverunit in the user's hand away from the body of the user as illustrated inFIG. 36. The field of view with reference to FIG. 36 is limited toplanes extending upward from a plane of sight 620 extending from theeyes 621 of the user 600 downward and intersecting a horizontal plane622 extending from or slightly above the user's waist substantially atarm's length as illustrated by the lower phantom position 604 in FIG.36. Acceptable planes in the field of view are those planes rotatedupward from plane of sight 620 from the lower phantom position 604 toand above the upper phantom position 608 illustrated in FIG. 36. Each ofthese upward extending planes beginning with the plane of sight 620extending from the eyes 621 of the user 600 to the lower phantomposition 604 to and above the upper phantom position 608 require theuser to hold the receiver unit 12' away from the body and atsuccessively higher positions which enhances reception by causing thereceiver unit to be positioned closer to a free space condition andfurther vertically upward within the range 611 for enhancing receiversensitivity by spacing the receiver away from the ground. Asillustrated, preferably nλ multiple wavelengths 612 space the receiverunit 12' away from the user 600 when the receiver unit is positioned inthe upwardly extending planes as described above while viewing thedisplay 424 to directionally track the transmitter 18.

As illustrated in FIG. 37, the position 608 of holding the housing 614of the receiver unit 12' is such that the display 424 for displaying thestrength of the received transmissions 610 from the transmitter 18during directional tracking is in the line of sight of the eyes 621 ofthe user 600 to the display 424 which is located in a recess 623 havingends defined by bottom 624 and an opening 626 within the housing 614. Ina preferred embodiment of the present invention, the display 424 issurface mounted on a circuit board (not illustrated) containing thereceiver electronics as described above. As can be seen from FIG. 37,the field of view limiter 618 is set in the housing 614 in front of thedirectional antenna 431 with reference to the line of sight extendingbetween eyes 621 of the user 600 and the at least one transmitter 18.

FIG. 38 illustrates an enlarged view of the field of view limiter 618.The field of view limiter 618 is set in the recess 623 in the housing614 in opening 626 extending inward from outer surface 628 of thehousing. The display 424 is located at the bottom 624 of the opening 626preferably as stated above as part of a surface mount on a circuit boardcontaining the receiver electronics. However, it should be understoodthat the invention is not limited to the surface mounting of the display424 on the circuit board of the receiver electronics.

The field of view is defined by a pair of lines 630 and 632 representinglight rays respectively extending from opposed edges 634 and 636 of thedisplay 424 to opposed edges 636 and 638 of the opening 626respectively. The angle 640 subtended by the straight lines 630 and 632may be as great as 45° with 30° or less being preferred. Theaforementioned angular ranges require the user 600 when holding thereceiver unit 12' in the user's hand 602 to position the receiver unitwithin an angular orientation such that the line of sight of the userextends from the user's eyes 621 permitting light rays 630 and 632 totravel from the bottom 624 of the opening 626 to the user's eyes 621.

With the configuration of the housing 614, as illustrated in FIG. 37,including the positioning of the switch 426, directional antenna 431 andthe field of view limiter 618 relative to the housing, the user 600 mustposition the receiver unit 12' between positions 604, 606 and 608 andabove away from the body and in planes at or above plane of sight 620 asillustrated in FIG. 36. With this set of spatial conditions, when thehand 602 of the user 600 holds the directional antenna activating switch426 in the position activating the directional antenna 431, thetransmissions 610 extend directly between the directional antenna of thereceiver unit 12' and the antenna of the transmitter 18, without signalattenuation or radio interference introduced by the holding of thereceiver unit in the hand 602 of the user 600 in the line of sightbetween the antennas of the receiver unit 12' and the at least onetransmitter unit 14, 16 and 18. Furthermore, the field of view limiter618 enhances reception by causing the user to hold the receiver unit 12in an elevated position away from the body of the user 600 to make thedisplay 424 visible.

While a preferred form of the field of view limiter 618 is asillustrated in FIG. 38, it should be understood that other optical ormechanical mechanisms or combinations thereof may be used, which preventthe user 600 from seeing the display 424 when the receiver unit 12' isheld in positions close to the body in a lowered position, with thepractice of the invention. Such field of view limiters could includeother optical elements such as lenses and/or reflective surfaces aloneor in combination with the recess 623 as illustrated in FIGS. 37 and 38.

While the invention has been described in terms of its preferredembodiments, it should be understood that numerous modifications may bemade thereto without departing from the spirit and scope of theinvention. For example, it should be understood that the presentinvention is not limited to the use of a particular type of processorwith a digital signal processor only being the preferred processor.Moreover, the present invention is not limited to the particular circuitdesigns or the particular integrated circuits shown herein. It isintended that all such modifications fall within the scope of theappended claims. ##SPC1##

What is claim is:
 1. A radio receiver unit for directionally tracking atleast one radio transmitter comprising:a housing containing a radioreceiver including a directional antenna for receiving radiotransmissions from the at least one radio transmitter; a display, whichis electrically coupled to the receiver and fixed in position withrespect to the housing, for visually displaying a strength of the radiotransmissions received by the directional antenna; and a field of viewlimiter for limiting light emanating from the display to a field of viewof the display when a user holds the receiver unit in the user's handaway from the body of the user, the field of view being limited toplanes extending upward from a plane of sight extending from the eyes ofthe user downward and intersecting a horizontal plane extending from theuser's waist substantially at arms length of the user.
 2. A radioreceiver unit in accordance with claim 1 wherein:the field of viewlimiter limits the field of view of the display by the user to planesextending upward from a plane of sight extending from the eyes of theuser downward and intersecting a horizontal plane extending from theuser's chest substantially at arms length of the user.
 3. A radioreceiver unit in accordance with claim 2 wherein:the field of viewlimiter is contained within the housing.
 4. A radio receiver unit inaccordance with claim 3 further comprising:a switch, coupled to thereceiver, the switch having a first position at which the directionalantenna is not operative to receive the radio transmissions from the atleast one radio transmitter and a second position at which thedirectional antenna is operative to receive the radio transmissions fromthe at least one radio transmitter, the switch being positioned relativeto the housing so that a hand of the user of the receiver duringdirectional tracking of the at least one transmitter holds the switch inthe second position; and wherein the directional antenna is positionedrelative to the housing so that during the holding of the switch in thesecond position by the user's hand a line of sight between thedirectional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.5. A receiver unit in accordance with claim 4 wherein:the field of viewlimiter is set in the housing in front of the directional antenna withreference to a line of sight extending between the user and the at leastone transmitter.
 6. A radio receiver unit in accordance with claim 3wherein the field of view limiter comprises:an opening within thehousing extending inward from an outer surface of the housing and havinga bottom with the display being positioned at the bottom.
 7. A radioreceiver unit in accordance with claim 6 wherein:the field of viewlimited by the field of view limiter is defined by a pair of straightlines respectively extending from opposed edges of the display toopposed edges of the opening.
 8. A radio receiver unit in accordancewith claim 7 further comprising:a switch, coupled to the receiver, theswitch having a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking of the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 9. A receiver unit in accordance withclaim 8 wherein:the field of view limiter is set in the housing in frontof the directional antenna with reference to a line of sight extendingbetween the user and the at least one transmitter.
 10. A radio receiverunit in accordance with claim 6 further comprising:a switch, coupled tothe receiver, the switch having a first position at which thedirectional antenna is not operative to receive the radio transmissionsfrom the at least one radio transmitter and a second position at whichthe directional antenna is operative to receive the radio transmissionsfrom the at least one radio transmitter, the switch being positionedrelative to the housing so that a hand of the user of the receiverduring directional tracking of the at least one transmitter holds theswitch in the second position; and wherein the directional antenna ispositioned relative to the housing so that during the holding of theswitch in the second position by the user's hand a line of sight betweenthe directional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.11. A receiver unit in accordance with claim 10 wherein:the field ofview limiter is set in the housing in front of the directional antennawith reference to a line of sight extending between the user and the atleast one transmitter.
 12. A radio receiver unit in accordance withclaim 2 further comprising:a switch, coupled to the receiver, the switchhaving a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 13. A receiver unit in accordancewith claim 12 wherein:the field of view limiter is set in the housing infront of the directional antenna with reference to a line of sightextending between the user and the at least one transmitter.
 14. A radioreceiver unit in accordance with claim 1 wherein:the field of viewlimiter is contained within the housing.
 15. A radio receiver unit inaccordance with claim 14 wherein the field of view limiter comprises:anopening within the housing extending inward from an outer surface of thehousing and having a bottom with the display being positioned at thebottom.
 16. A radio receiver unit in accordance with claim 15wherein:the field of view limited by the field of view limiter isdefined by a pair of straight lines respectively extending from opposededges of the display to opposed edges of the opening.
 17. A radioreceiver unit in accordance with claim 16 further comprising:a switch,coupled to the receiver, the switch having a first position at which thedirectional antenna is not operative to receive the radio transmissionsfrom the at least one radio transmitter and a second position at whichthe directional antenna is operative to receive the radio transmissionsfrom the at least one radio transmitter, the switch being positionedrelative to the housing so that a hand of the user of the receiverduring directional tracking of the at least one transmitter holds theswitch in the second position; and wherein the directional antenna ispositioned relative to the housing so that during the holding of theswitch in the second position by the user's hand a line of sight betweenthe directional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.18. A radio receiver unit in accordance with claim 15 furthercomprising:a switch, coupled to the receiver, the switch having a firstposition at which the directional antenna is not operative to receivethe radio transmissions from the at least one radio transmitter and asecond position at which the directional antenna is operative to receivethe radio transmissions from the at least one radio transmitter, theswitch being positioned relative to the housing so that a hand of theuser of the receiver during directional tracking of the at least onetransmitter holds the switch in the second position; and wherein thedirectional antenna is positioned relative to the housing so that duringthe holding of the switch in the second position by the user's hand aline of sight between the directional antenna and the at least one radiotransmitter is not occluded by the user's hand holding the switch in thesecond position.
 19. A receiver unit in accordance with claim 18wherein:the field of view limiter is set in the housing in front of thedirectional antenna with reference to a line of sight extending betweenthe user and the at least one transmitter.
 20. A radio receiver unit inaccordance with claim 14 further comprising:a switch, coupled to thereceiver, the switch having a first position at which the directionalantenna is not operative to receive the radio transmissions from the atleast one radio transmitter and a second position at which thedirectional antenna is operative to receive the radio transmissions fromthe at least one radio transmitter, the switch being positioned relativeto the housing so that a hand of the user of the receiver duringdirectional tracking of the at least one transmitter holds the switch inthe second position; and wherein the directional antenna is positionedrelative to the housing so that during the holding of the switch in thesecond position by the user's hand a line of sight between thedirectional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.21. A receiver unit in accordance with claim 20 wherein:the field ofview limiter is set in the housing in front of the directional antennawith reference to a line of sight extending between the user and the atleast one transmitter.
 22. A radio receiver unit in accordance withclaim 1 further comprising:a switch, coupled to the receiver, the switchhaving a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking of the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 23. A receiver unit in accordancewith claim 22 wherein:the field of view limiter is set in the housing infront of the directional antenna with reference to a line of sightextending between the user and the at least one transmitter.
 24. A radioreceiver unit for directionally tracking at least one radio transmittercomprising:a housing containing a radio receiver including a directionalantenna for receiving radio transmissions from the at least one radiotransmitter; a display, which is electrically coupled to the receiverand fixed in position with respect to the housing, for visuallydisplaying a strength of the radio transmissions received by thedirectional antenna from the at least one radio transmitter; and a fieldof view limiter for limiting light emanating from the display to a fieldof view of the display wherein in order to view the display a user holdsthe receiver unit in the user's hand away from the body of the user andat least at waist height and above, the field of view subtending anangle not greater than 45°, the angle being defined by a pair of linesrespectively extending between opposed edges of the display and edges ofthe field of view limiter.
 25. A radio receiver unit in accordance withclaim 24 wherein:the angle is not greater than 30°.
 26. A radio receiverunit in accordance with claim 25 wherein:the field of view limiter iscontained within the housing.
 27. A radio receiver unit in accordancewith claim 26 further comprising:a switch, coupled to the receiver, theswitch having a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking of the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 28. A receiver unit in accordancewith claim 27 wherein:the field of view limiter is set in the housing infront of the directional antenna with reference to a line of sightextending between the user and the at least one transmitter.
 29. A radioreceiver unit in accordance with claim 26 wherein the field of viewlimiter comprises:an opening within the housing extending inward from anouter surface of the housing and having a bottom with the display beingon the bottom.
 30. A radio receiver unit in accordance with claim 29wherein:the field of view limited by the field of view limiter isdefined by a pair of straight lines respectively extending from opposededges of the display to opposed edges of the opening.
 31. A radioreceiver unit in accordance with claim 30 further comprising:a switch,coupled to the receiver, the switch having a first position at which thedirectional antenna is not operative to receive the radio transmissionsfrom the at least one radio transmitter and a second position at whichthe directional antenna is operative to receive the radio transmissionsfrom the at least one radio transmitter, the switch being positionedrelative to the housing so that a hand of the user of the receiverduring directional tracking of the at least one transmitter holds theswitch in the second position; and wherein the directional antenna ispositioned relative to the housing so that during the holding of theswitch in the second position by the user's hand a line of sight betweenthe directional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.32. A receiver unit in accordance with claim 31 wherein:the field ofview limiter is set in the housing in front of the directional antennawith reference to a line of sight extending between the user and the atleast one transmitter.
 33. A radio receiver unit in accordance withclaim 29 further comprising:a switch, coupled to the receiver, theswitch having a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking of the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 34. A receiver unit in accordancewith claim 33 wherein:the field of view limiter is set in the housing infront of the directional antenna with reference to a line of sightextending between the user and the at least one transmitter.
 35. A radioreceiver unit in accordance with claim 25 further comprising:a switch,coupled to the receiver, the switch having a first position at which thedirectional antenna is not operative to receive the radio transmissionsfrom the at least one radio transmitter and a second position at whichthe directional antenna is operative to receive the radio transmissionsfrom the at least one radio transmitter, the switch being positionedrelative to the housing so that a hand of the user of the receiverduring directional tracking of the at least one transmitter holds theswitch in the second position; and wherein the directional antenna ispositioned relative to the housing so that during the holding of theswitch in the second position by the user's hand a line of sight betweenthe directional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.36. A receiver unit in accordance with claim 35 wherein:the field ofview limiter is set in the housing in front of the directional antennawith reference to a line of sight extending between the user and the atleast one transmitter.
 37. A radio receiver unit in accordance withclaim 24 wherein:the field of view limiter is contained within thehousing.
 38. A radio receiver unit in accordance with claim 37 whereinthe field of view limiter comprises:an opening within the housingextending inward from an outer surface of the housing and having abottom with the display being positioned at the bottom.
 39. A radioreceiver unit in accordance with claim 38 wherein:the field of viewlimited by the field of view limiter is defined by a pair of straightlines respectively extending from opposed edges of the display toopposed edges of the opening.
 40. A radio receiver unit in accordancewith claim 39 further comprising:a switch, coupled to the receiver, theswitch having a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking of the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 41. A receiver unit in accordancewith claim 40 wherein:the field of view limiter is set in the housing infront of the directional antenna with reference to a line of sightextending between the user and the at least one transmitter.
 42. A radioreceiver unit in accordance with claim 38 further comprising:a switch,coupled to the receiver, the switch having a first position at which thedirectional antenna is not operative to receive the radio transmissionsfrom the at least one radio transmitter and a second position at whichthe directional antenna is operative to receive the radio transmissionsfrom the at least one radio transmitter, the switch being positionedrelative to the housing so that a hand of the user of the receiverduring directional tracking of the at least one transmitter holds theswitch in the second position; and wherein the directional antenna ispositioned relative to the housing so that during the holding of theswitch in the second position by the user's hand a line of sight betweenthe directional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.43. A receiver unit in accordance with claim 42 wherein:the field ofview limiter is set in the housing in front of the directional antennawith reference to a line of sight extending between the user and the atleast one transmitter.
 44. A radio receiver unit in accordance withclaim 37 further comprising:a switch, coupled to the receiver, theswitch having a first position at which the directional antenna is notoperative to receive the radio transmissions from the at least one radiotransmitter and a second position at which the directional antenna isoperative to receive the radio transmissions from the at least one radiotransmitter, the switch being positioned relative to the housing so thata hand of the user of the receiver during directional tracking of the atleast one transmitter holds the switch in the second position; andwherein the directional antenna is positioned relative to the housing sothat during the holding of the switch in the second position by theuser's hand a line of sight between the directional antenna and the atleast one radio transmitter is not occluded by the user's hand holdingthe switch in the second position.
 45. A receiver unit in accordancewith claim 44 wherein:the field of view limiter is set in the housing infront of the directional antenna with reference to a line of sightextending between the user and the at least one transmitter.
 46. A radioreceiver unit in accordance with claim 24 further comprising:a switch,coupled to the receiver, the switch having a first position at which thedirectional antenna is not operative to receive the radio transmissionsfrom the at least one radio transmitter and a second position at whichthe directional antenna is operative to receive the radio transmissionsfrom the at least one radio transmitter, the switch being positionedrelative to the housing so that a hand of the user of the receiverduring directional tracking of the at least one transmitter holds theswitch in the second position; and wherein the directional antenna ispositioned relative to the housing so that during the holding of theswitch in the second position by the user's hand a line of sight betweenthe directional antenna and the at least one radio transmitter is notoccluded by the user's hand holding the switch in the second position.47. A receiver unit in accordance with claim 46 wherein:the field ofview limiter is set in the housing in front of the directional antennawith reference to a line of sight extending between the user and the atleast one transmitter.